The effect of piperine on the fertilization of eggs with sperm was investigated in female hamsters. They were intragastrically treated with piperine at doses of 50 and 100mg/kg BW from day 1 through day 4 of the oestrous cycle. During piperine treatment, these females were superovulated and artificially inseminated (AI) with spermatozoa from untreated male hamsters at 12h after hCG injection. The fertilization and growth of embryos were examined at various times after AI. In control hamsters, the percent fertilization increased with time, from 27.4±3.3% at 9h after AI to 75.3±9.6 at 24h after AI. Administration of piperine to the superovulated animals markedly enhanced the percent fertilization at 9h after AI. It was increased to 85.4±4.1 and 82.8±4.8% by piperine at doses of 50 and 100mg/kg BW, respectively. However, examination of the embryos retrieved 48h after AI revealed no differences in the stage of embryonic development among different groups of animals. The possibility that this effect was due to the direct action of vanillic acid, a major piperine metabolite, was testedin vitroDirect exposure of spermatozoa to vanillic acid at doses 25–100mg% did not significantly affect their motility, percent acrosome reaction or fertilizing ability. This suggests that the enhancement of fertilization by piperine treatment was not related to the secretion of vanillic acid into the oviduct.