Summary. Domestic cats experiencing a natural or FSH-induced oestrus were studied. Mated cats produced fewer (P < 0·01) unfertilized oocytes and more (P < 0·01) morulato blastocyst-stage embryos of better quality after a natural oestrus than after FSH treatment. Serum oestradiol- 17β concentrations were lower (P < 0·05) and progesterone levels rose earlier (P < 0·05) in the induced oestrus compared to the natural oestrus group. Morula/blastocyst-stage embryos from both groups transferred to 15 FSH/hCG-treated recipients produced 3 pregnancies and 2 live-born litters (1 from a natural oestrus donor and 1 from an FSH-treated donor). These results indicate that fertilization rates and embryo quality in domestic cats appear to be compromised by the FSH treatment, probably because of altered oestradiol-17β and progesterone concentrations.