The incidence of ovulation and subsequent pregnancy in post partum marmosets were studied. By two weeks following delivery, all the animals (N=10) had ovulated as determined by a gradual rise in plasma estradiol followed by a rise in plasma progesterone (p>31.8 nmol/l) levels. The mean day of ovulation was 11.3±1.5 (9–14 days). Eighty percent of these females became pregnant as indicated by a continued rise in plasma progesterone levels (p>127 nmol/l) and as confirmed by transabdominal uterine palpation. These results suggest that the day of delivery could serve as a useful parameter to monitor ovulation and pregnancy in this non-menstruating primate species.