Comparisons were made between 2 methods of oocyte recovery from the ovarian follicles of slaughtered mares: 500 oocytes (3 per mare) were obtained by aspiration of follicular fluid from ovaries of 162 mares, and 120 oocytes (8 per mare) by isolation and rupture of follicles from ovaries of 14 mares. In the oocytes recovered after rupture of follicles, 89.2% were morphologically unchanged, in comparison to 29.3% obtained by aspiration of follicular fluid. Stallion spermatozoa capacitated in vitro were tested on zona-free hamster oocytes. The stallion spermatozoa were washed in TCM-199 and preincubated for 4-5 h at a concentration of 0.5-1.0 x 10(9)/ml. Of the 305 hamster oocytes fertilized by spermatozoa obtained from 8 stallions, 77.8% penetration was confirmed on the basis of the presence of enlarged heads of spermatozoa and male pronuclei with their corresponding tails. Spermatozoa from one of the stallions were tested 10 times, 8 months later. The percentage of penetrated oocytes averaged 52.8% (39.1-76.9%). In the control group of oocytes preincubated in the absence of spermatozoa, spontaneous activation and female pronuclei were found in 12 out of a total of 57.