4,000,000 SECURED DEMAND PROMISSORY NOTE

Exhibit 10.23

$4,000,000 SECURED DEMAND PROMISSORY NOTE

     FOR VALUE RECEIVED, the undersigned, MERCANTILE BANCORP, INC., a Delaware corporation (“Mercantile”), jointly and severally, promises to pay to the order of GREAT RIVER BANCSHARES, INC., a Nevada corporation (“ Lender ”), ON DEMAND, at 8620 West Tropicana, Las Vegas, Nevada 89180 or such other place as may be designated by the holder of this Note, the principal sum of FOUR MILLION AND 00/100 DOLLARS ($4,000,000.00) or so much thereof as may be advanced and outstanding from time to time under this $4,000,000 Secured Demand Promissory Note (this “ Note ”), together with interest on said principal sum from the date hereof until this Note is paid in full, at the Interest Rate. As used herein, the term “ Interest Rate ” shall mean, until the occurrence of an event described in Paragraph 7 below, when the Interest Rate shall thereafter equal the Default Rate, a per annum rate of interest equal to seven and one-half percent (7.50%).

     1 . Payment . The principal sum of this Note and interest thereon shall be paid as follows:

     (a) Interest at the Interest Rate shall accrue on the principal amount of this Note from the date hereof and shall be payable on the first (1st) day of each month, beginning with March 1, 2009.

     (b) If not sooner paid, the principal sum of this Note, together with accrued interest thereon, shall be repaid immediately upon Lender’s written demand. The Borrower acknowledges and agrees that Lender may require immediate repayment of either all or a portion of the indebtedness evidenced hereby.

     2.  Late Charge; Default Rate . In the event that any scheduled monthly installment of principal and/or interest under this Note is received by Lender more than two (2) days after the same is due, Borrower shall pay to Lender a late charge equal to five percent (5.0%) of such delinquent payment. In the event that an Event of Default (as defined below) shall occur and Lender shall exercise its right to declare this Note to be due and payable as set forth in Paragraph 7 below, then the unpaid principal balance under this Note shall thereafter bear interest at the Default Rate. As used herein, the term “ Default Rate ” shall mean a per anum rate of interest equal to twelve and one-half percent (12.5%).

     3.  Prepayment . The unpaid principal balance of this Note may be prepaid in whole or in part, at any time and from time to time, without prepayment charge or penalty. All prepayments shall be applied first to all charges and payments due from Borrower to Lender under this Note other than principal and interest, second to accrued and unpaid interest, and third to principal.

     4.  Payments and Computations . All payments on account of indebtedness evidenced by this Note shall be made not later than 2:00 P.M. (Central time) on the day when due in lawful money of the United States and shall be first applied to all charges and payments due from Borrower to Lender under this Note other than principal and interest, second to interest on the

unpaid principal balance of this Note and the remainder to principal. All computations of interest shall be made by Lender on the actual days outstanding on the basis of a three hundred sixty (360) day calendar year. Said payments are to be made via wire transfer of funds to Lender’s office at 8620 West Tropicana, Las Vegas, Nevada 89180 or at such place as Lender or the legal holder of this Note may, from time to time, in writing appoint. Unless otherwise specified herein, all interest payable under this Note is paid in arrears.

     5.  Applicable Laws . This Note shall be construed and enforced in accordance with the laws of the State of Illinois and shall be conclusively deemed for all purposes to have been executed and delivered in the State of Illinois for performance therein. This Note is given for an actual loan of money for business purposes and is not for agricultural, consumer, personal or residential purposes.

     6.  Collateral Security . The payment of this Note and all obligations of Borrower in connection with this Note are secured by that certain Stock Pledge Agreement (Borrower), dated as of the date hereof, by and between Lender and Borrower (the “ Pledge Agreement ”) and by that certain Collateral Assignment of Promissory Note and Other Loan Documents, dated as of the date hereof, by and between Lender and Borrower (the “ Collateral Assignment ”).

     7.  Event of Default . An “ Event of Default ” hereunder shall be deemed to have occurred (i) if Borrower shall fail to pay when due any sum of money due and owing under this Note; (ii) Borrower uses the proceeds from this Note for any purposes other than purchasing a $4,000,000.00 promissory note executed by Mid-American Bancshares, Inc. (“Mid-America”), Robert S. Wholey and Janet F. Wholey in favor of First Community Bank, a Missouri state chartered bank (“First Community”); (iii) Borrower breaches any covenant, representation or warranty set forth in this Note, the Pledge Agreement or the Collateral Assignment; (iv) an &ldq