A software lock-in amplifier (SLIA) was developed to allow high-time-resolution measurement of membrane capacitance as a single-cell assay of exocytosis. The unique feature of this "virtual instrument" is that it is thoroughly integrated with a computer-controlled patch-clamp amplifier (EPC-9) to allow estimation of equivalent circuit parameters based upon calibrated admittance measurements rather than just relative changes. Since the same software package ("PULSE") controls both the EPC-9 and the SLIA, instrument settings which affect admittance calculations (gain, filtering, etc.) are always "known" by the SLIA. Attenuation and phase shifts introduced within the EPC-9 by low-pass filters and other circuitry are modelled and automatically corrected by the software. In addition, changes in the measured signal introduced by whole-cell capacitance and series resistance compensation are accounted for. The noise of capacitance measurements is nearly optimal and resistive parameters can vary over a large range without inducing artifactual changes in capacitance estimates.