Effective monetary policy requires a good understanding of banks’ demand for liquidity also called “reserves” at the central bank. This paper models a reserve demand function for the Philippines with the dual goal of: (i) estimating the slope or the elasticity of rates to reserve shocks; and (ii) identifying transition points that define scarce, ample, and abundant reserves. Results show evidence of a non-linear, time-varying slope for the reserve demand function, as suggested in theory. This elasticity of rates to reserves was found to be generally negative but close to zero throughout the study period, which suggests that Philippine banks have maintained a generally ample level of reserves over time. The slope of the demand function also shifted along with structural changes in both rates (i.e., banks’ balance-sheet costs and other frictions) and reserves (i.e., regulation or market functioning). The adoption of the Interest Rate Corridor (IRC) framework in 2016 encouraged more active monetary operations by the BSP and more active liquidity management by Philippine banks, which supported a tighter relationship between reserves and rates. Liquidity-enhancing measures deployed during the pandemic pushed reserve levels into abundant territory, which brought elasticity back to near-zero levels. As conditions normalized, the current recovery period saw reserve levels declining and pushed reserve demand back into ample and scarce territory. In terms of policy implications, this paper’s methodology can be used in real-time to assess the ampleness of reserves, which in effect, allows policymakers to gauge the extent of their control over short-term market interest rates.