This study demonstrates for the first time, that membrane vesicles originated from the hamster sperm head after the occurrence of the acrosome reaction, possess specific strychnine binding sites. [ 3 H]Strychnine binding was saturable and reversible, being displaced by unlabeled strychnine (IC 5 0 =26.7+/-2.3μM). Kinetic analysis revealed one binding site with K d =120nM and B m a x =142fmol/10 6 spermatozoa. Glycine receptor agonists β-alanine and taurine inhibited strychnine binding by 20-30%. Surprisingly, glycine stimulated binding by about 40-50%. Results obtained in this study strongly suggest the presence of glycine receptors-with distinctive kinetic properties on the periacrosomal plasma membrane of hamster spermatozoa. Localization of this receptor fits well with its previously proposed role in acrosomal exocytosis during mammalian fertilization.