In rats fertilized during the 1 st or 2nd day postpartum (second consecutive pregnancy), suckling induces intrauterine blastocyst dormancy and delays implantation for 8–22 days. In spite of this relatively long time of blastocyst dormancy, associated with greatly reduced cellular metabolism, subsequent implantation and growth results in significantly larger litters (13.6 ± 0.62; x ± SE) than in primigravidas (10.4 ± 0.89) or in rats with a second-spaced pregnancy (11.2 ± 0.72). Enhanced fecundity in rats with second consecutive pregnancies may be attributed to increased ovarian blood supply of preceding gestation and/or to augmented pituitary FSH-LH secretion resulting in intensified ovarian gonadotropic stimulation for postpartum ovulation.