With a specially constructed chamber, Ca2+ uptake by mouse spermatozoa was monitored continuously during capacitation and the acrosome reaction. It was shown, using calcium ion-selective microelectrodes, that there was an initial uptake of Ca2+ by spermatozoa undergoing capacitation. Such net transport was also promoted by the divalent cation ionophores A23187 or ionomycin. An anion inhibitor, SITS, produced dose-dependent inhibition of Ca2+ uptake. This inhibitor reduced the incidence of capacitation as revealed by a reduction in the B pattern by chlortetracycline (CTC) assay and thus inhibited fertilization, suggesting that anions are involved in calcium uptake in mouse spermatozoa.