The sperm acrosome reaction takes place in response to progesterone and zona pellucida. Progesterone may act on more than one type of surface receptor, of which one is a gamma-aminobutyric acid (GABA) type A-like receptor. Although there is direct evidence of GABA initiation of mouse sperm acrosome reaction, there are conflicting results regarding GABA-induced exocytosis in human spermatozoa. We have examined whether GABA would initiate exocytosis in human spermatozoa using the chlortetracycline assay and a zona-free hamster oocyte test. Human spermatozoa preincubated for > or = 3 h in Biggers-Whitten-Whittingham medium with 0.35% bovine serum albumin underwent acrosome reactions in response to GABA, with maximal responses in spermatozoa preincubated for 9 h. The effect was concentration-dependent. Preincubated spermatozoa treated with GABA were able to fertilize a higher proportion of zona-free oocytes, with a higher number of spermatozoa penetrating each oocyte. Exposure of preincubated spermatozoa to GABA and progesterone together resulted in a higher proportion of acrosome reactions than when each agonist was used alone. The effect of GABA was mediated by the influx of extracellular Ca2+ because inclusion of EGTA or the Ca2+ channel antagonist La3+ prevented GABA-induced acrosome reactions. These results indicate that GABA can initiate exocytosis in capacitated human spermatozoa and open up possibilities for studies of signalling mechanisms activated upon occupancy of the GABAA receptor present on the sperm surface.