Corpus luteum (CL) progesterone concentration and total progestins in the peripheral plasma were measured at various stages of pregnancy in 18 young rabbits 35 weeks old and 26 aged rabbits 250 weeks old. Fertilization rate, average number of CL, and viable fetuses in the young does were 83, 7.7, and 5.8%, respectively. Corresponding values in the aged does were 81, 6.2, and 1.1%. There were no differences between young and aged does in any of the parameters of ovarian function measured until after Day 12 of pregnancy. This was true even though five of nine aged animals did not have viable fetuses at this time. Significant differences were detected in aged animals without viable fetuses only at Day 24. At this time average CL weight (6.3 mg) total luteal tissue (30.9 mg), and total CL progesterone content (0.49 µg) in aged animals without viable fetuses were significantly (P < 0.05) less than corresponding values in aged animals with viable fetuses (15.1 mg, 110.6 mg, 2.55 µg) and in young animals all with viable fetuses (16.5 mg, 120.7 mg, 2.83 µg). These latter two groups did not differ. Peripheral plasma progestins on Day 24 of planned pregnancy in aged animals without viable fetuses (3.2 ng/ml) were also significantly (P < 0.05) less than in aged animals with viable fetuses (16.8 ng/ml) and in young animals (11.4 ng/ml). Again, these latter two groups did not differ. It is concluded that the decrease in fecundity in aged animals is not caused by a lack of progestin secretion, but rather the absence of viable fetuses may cause luteal regression. Thus, reproductive failure in aged does appears to be directly associated with uterine dysfunction.