Intracellular recordings and injections of procion yellow (PY) were made in parvocellular neurons in hypothalamic slices of female guinea pigs. Eighty-five neurons, with an average resting membrane potential of -35 mV, were recorded in the arcuate (ARC) ventromedial (VM), and in the cell-poor zones between the ARC and VM. Eleven of the ARC neurons and four neurons from the cell-poor zone could be driven antidromically by median eminence (ME) stimulation, nine other neurons from the three areas could be driven orthodromically by stria terminalis (ST) stimulation. Twenty-eight parvocellular neurons were tested with 17 beta-estradiol (E2), which was applied in the bathing medium as the free steroid. Eleven neurons (nine ARC and two cell-poor-zone neurons) were hyperpolarized 2 to 24 mV by 10(-10) M E2 concentrations. 10(-8) M estrone concentration was without effect on three of these cells. Through the intracellular injection of PY, the estrogen-sensitive neurons (N = 11) were identified as small fusiform cells with few dendrites. Spine-like appendages were found on only one of these cells. None of the larger pyramidal-like neurons of these areas responded to the application of E2.