For this study, we investigated the changes in the electrophysiological parameters of Sertoli cells in seminiferous tubules from 17 - 19 day-old rats induced by testosterone. Using conventional intracellular microelectrode techniques, we analysed the membrane potential and its input resistance. The entire tubules were fixed in a superfusion chamber continuously perfused with Krebs-Ringer bicarbonate buffer (pH 7.4, 32 degrees C). Visual control of cell impalement was achieved using an inverted microscope. The parameters analysed were passed through an amplifier and recorded using a proprietary software system. The topical application of testosterone (0.1 to 10 microM) led to an immediate (within 30 seconds) and significant dose-dependent depolarization of the membrane potential of the cell at all concentrations used. Concomitantly, the input resistance of the cell membrane underwent a significant increment at 30 seconds. These changes returned to resting values after washout. Topical administration of 17beta-estradiol or progesterone (10 microM) did not change the membrane potential. The addition of the K +ATP channel agonist diazoxide to the perfusion buffer nullified the depolarization effect of testosterone at 30 seconds. This result suggests that the immediate action of testosterone is associated with the closing of K +ATP channels, thereby depolarizing the membrane.