This paper analyzes interbank markets under currency boards. Under such an environment, problematic endogeneity issues common to other monetary regimes do not arise. Using daily data from the interbank markets in Bulgaria and Lithuania we show, that contrary to the existing literature, overnight interest rates tend to decrease towards the end of the reserve holding period. Empirical results are supported by a finite horizon heterogeneous agents model showing that interest rates tend to decrease in the case of excess aggregate reserves in the banking system. Results contrast with Quir'os and Mendiz'abal (2006) who find that interest rates should be increasing regardless of the outstanding aggregate liquidity in the market. We also show that responsiveness of banks to interest rate changes diminishes as the end of reserve holding period approaches. Under certain circumstances this could lead to multiple equilibria with increasing or decreasing interest rates.