1. Perforated patch-clamp recordings were made from the three major classes of hippocampal neurons in conventional in vitro slices prepared from adult guinea pigs. This technique provided experimental estimates of passive membrane properties (input resistance, RN, and membrane time constant, tau m) determined in the absence of the leak conductance associated with microelectrode impalement or the washout of cytoplasmic constituents associated with conventional whole-cell recordings. 2. To facilitate comparison of our data with previous results and to determine the passive membrane properties under conditions as physiological as possible, recordings were made at the resting potential, in physiological saline, and without any added blockers of voltage-dependent conductances. 3. Membrane-potential responses to current steps were analyzed, and four criteria were used to identify voltage responses that were the least affected by activation of voltage-dependent conductances. tau m was estimated from the slowest component (tau 0) of multiexponential fits of responses deemed passive by these criteria. RN was estimated from the slope of the linear region in the hyperpolarizing direction of the voltage-current relation. 4. It was not possible to measure purely passive membrane properties that were completely independent of membrane potential in any of the three classes of hippocampal neurons. Changing the membrane potential by constant current injection resulted in changes in RN and tau 0; subthreshold depolarization produced an increase, and hyperpolarization a decrease, in both RN and tau 0 for all three classes of hippocampal neurons. 5. Each of the three classes of hippocampal neurons also displayed a depolarizing "sag" during larger hyperpolarizing voltage transients. To evaluate the effect of the conductances underlying this sag on passive membrane properties, 2-5 mM Cs+ was added to the physiological saline. Extracellular Cs+ effectively blocked the sag in all three classes of hippocampal neurons, but the effect of Cs+ on RN, tau 0, and the voltage dependence of these parameters was unique for each class of neurons. 6. CA1 pyramidal neurons had an RN of 104 +/- 10 (SE) M omega and tau 0 of 28 +/- 2 ms at a resting potential of -64 +/- 2 mV (n = 12). RN and tau 0 were larger at more depolarized potentials in these neurons, but the addition of Cs+ to the physiological saline reversed this voltage dependence. 7. CA3 pyramidal neurons had an RN of 135 +/- 8 M omega and tau 0 of 66 +/- 4 ms at a resting potential of -64 +/- 1 mV (n = 14).(ABSTRACT TRUNCATED AT 400 WORDS)