Intracellular recordings of magnocellular neurons from the supraoptic nucleus of guinea-pigs were made with KCI/K citrate- and biocytin-filled electrodes. Fifty of 99 cells exhibited a time-dependent inward rectification (TDR). The TDR was activated during hyperpolarizing current pulses to membrane potentials more hyperpolarized than -75 mV. In voltage-clamp recordings, an inward current appeared at voltage steps more hyperpolarized than -75 mV, with properties similar to the slow inward rectifier (I(h)) described in other tissues. The I(h) was blocked by 2 mM CsCI. BaCI(2) (100 to 500 muM) did not block the I(h). Immunocytochemical identification of the recorded cells revealed that both vasopressin (AVP)- and oxytocin (OT)- containing neurons exhibited an I(h).