It is known that Cl- is required for the zona pellucida (zona)- and progesterone-initiated mammalian sperm acrosome reaction (AR). Recent evidence indicates the involvement of a unique progesterone receptor resembling a GABAA receptor/Cl- channel in the latter event. In the study reported here, we investigated whether the in vitro porcine sperm AR initiated by heat-solubilized porcine zonae involves a different receptor/Cl- channel than does the progesterone-initiated AR. Capacitated porcine sperm failed to undergo the zona- and progesterone-initiated AR when suspended in a medium deficient in Cl-. We then employed 1) strychnine, an antagonist of neuronal glycine receptor/Cl- channels at nanomolar concentrations and of GABAA receptor/Cl- channels at micromolar concentrations and 2) (+)-bicuculline, an antagonist of neuronal GABAA receptor/Cl- channels that inhibits glycine receptor/Cl- channels only at higher concentrations. We report here that the zona-initiated AR was inhibited by 50 nM strychnine, but the progesterone-initiated AR required 1 microM for inhibition. Moreover, 1 microM (+)-bicuculline inhibited the progesterone-initiated AR, but the zona-initiated AR required 50 microM for inhibition. In addition, glycine initiated the porcine sperm AR, and that event was inhibited by strychnine and (+)-bicuculline with the same order of potency observed for the zona-initiated AR. Finally, glycine initiated the AR in human sperm, and that initiation was inhibited by 50 nM strychnine. This work provides the first evidence suggesting that a glycine receptor/Cl- channel is involved in the zona-initiated mammalian sperm AR.