Summary. Nulliparous, multiparous and post-partum rabbits were mated following treatment with luteinizing hormone or superovulation injections. A total of 1009 blastocysts was recovered 144 hr post coitum and they were measured microscopically. Ovulation rate tended to increase with increase in parity. Superovulatory response was not lower in post-partum animals. The number of degenerating blastocysts per doe increased in all superovulated groups. The percentage of viable blastocysts among the total number of recovered blastocysts did not vary in ovulated groups and in one superovulated group. A high percentage of oversized blastocysts was recorded in a superovulated group with a good response to gonadotrophins. Many undersized blastocysts were recorded for superovulated does with a poor response to gonadotrophins. Blastocyst size tended to decrease in does with a poor superovulatory response. The diameter of the embryonic disc following superovulation was smaller than in ovulated groups. There was interlitter and intralitter variation in the microscopical appearance of the embryonic disc. In pooled data the outer diameter of the blastocyst was not significantly correlated with the diameter of the embryonic disc. The position of the blastocysts in utero was not correlated with the diameter of the blastocyst or the diameter of the embryonic disc.