The accumulation of lipophilic cation radiolabeled tri-phenylmethyl phosphonium (TPMP+) was utilized to determine the resting membrane potential across the plasma membrane psi. Washed sperm of mammalian was suspended and incubated in low-K+ and high-K+ medium and allowed to take up the cation to a steady-state (20 min at 37 degrees C). By using this differential, the value obtained was transformed at value concentration using the intracellular volume and with these values inserted in the Nernst equation, we obtained the psi. During the maturation process the psi obtained in sperm cells of caput of the epididymis of New Zealand white rabbits was -75(+/-)-3 mV, in sperm cells of corpus and cauda, the psi obtained were -60(+/-)-5 and -52 +/- 3.5 mV, respectively. In the ejaculated spermatozoa the psi obtained was -69 +/- 2 mV. Finally, during the capacitation of human spermatozoa induced by the addition of calcium ions and the ionophore A23187 the psi obtained was -35 +/- 4 mV. These changes in the psi are significant because when the mammalian sperm cells are under different physiological conditions the membrane polarity is modified for the hyperpolarization and depolarization observed.