This study determined (1) the temporal kinetics of nuclear maturation at and after follicular oocyte aspiration and (2) the significance of variations in nuclear maturation and sperm-oocyte co-incubation interval on in vitro fertilization (IVF) in the domestic cat. Female cats were treated with pregnant mares' serum gonadotropin followed 84 h later with human chorionic gonadotropin (hCG). Twenty-five hours after hCG, ovarian follicular oocytes were aspirated laparoscopically and classified according to morphological criteria. Oocytes classified as mature were cultured for 0, 2, 4, 6, or 8 h in Ham's F10 medium and then stained for nuclear maturation. The proportion of oocytes at metaphase II was similar (P > 0.05) at 0 (38.2%) and 2 (48.5%) h of culture, but was higher (P < 0.05) at 4 (76.5%), 6 (82.8%), and 8 (92.3%) h. Oocytes also were collected and cultured as described above and then co-incubated for 2 or 18 h with 5 x 10(5) motile cat sperm cells/ml. The incidence of IVF (cleavage to 2-cells by 30 h post-insemination) was consistently 2-fold higher (P > 0.05) for the 18 h compared to the 2 h gamete co-incubation groups. However, despite differences in nuclear maturation at the time of oocyte recovery, IVF success was unaffected (P > 0.05) by pre-insemination culture interval (0 h, 72.7%; 2 h, 73.3%; 4 h, 70.7%; 6 h, 57.8%; 8 h, 62.5%; 18 h group). Thus, the majority of oocytes recovered from cats treated with this gonadotropin regimen has not achieved metaphase II by follicular aspiration.(ABSTRACT TRUNCATED AT 250 WORDS)