Does the federal funds rate respond to shocks when aggregate reserves are in the trillions of dollars? Has banks’ demand for reserves moved over time? We provide a structural time-varying estimate of the slope of the reserve demand curve over 2010-21. We estimate a time-varying vector autoregressive model at daily frequency with an instrumental variable approach to address endogeneity. Consistent with economic theory, our estimates show a nonlinear demand function that exhibits a negative slope in 2010-11 and 2018-19 but is flat over 2012-17 and after mid-2020. We also find that the curve has moved outward, both vertically and horizontally.