The accumulation of the lipophilic cation radiolabeled triphenylmethylphosphonium (TPMP+) was utilized to determine the resting membrane potential across the plasma membrane (psi) on human sperm. Washed sperm were suspended and incubated in low-K+ and high-K+ medium and allowed to take up the cation to a steady state (that is, 20 min at 37 degrees C). The data were transformed according to concentration using the intracellular volume and with these values inserted in the Nernst equation we obtain psi. When the concentration of K+ was low in the medium, the psi was -69 +/- 2 mV, the psi was temperature- and pH-dependent. The sperm were incubated in the same conditions in the presence of testosterone, estradiol, and progesterone. The addition of these hormones induced the decrease in psi by about 29%, 32%, and 40%, respectively. On the other hand, capacitation of spermatozoa was induced by the addition of calcium and the ionophore A23187 and in these conditions the psi decreased 50%. The action of the hormones on sperm membrane was accompanied by some steroid-dependent structural modification promoting the psi decrease. This effect plays an important role in the capacitation, preparing it for sperm penetration in the ova.