Unilateral ovariectomy (ULO) was carried out on rats at Day 8 of gestation: term is Day 23. The effect on peripheral plasma progesterone levels at Day 16 and on growth and cellular changes in the remaining corporalutea (CL) at Day 17 was determined by comparison with results from control and sham-operated rats. ULO increased the volume of remaining CL by 18% at Day 17. At Day 16 this effect was only apparent in rats with three or fewer CL remaining. CL hypertrophy was due to an increase in the volume of individual luteal cells and not to cell hyperplasia. Luteal cell nuclear volume also increased as did the number of endothelial cells per corpus luteum. The proportion of the corpus luteum occupied by vascular space did not alter. ULO had little effect on mean plasma progesterone levels at Day 16 (92 +/- 7 ng/ml treated; 86 +/- 11 ng/ml controls). In rats with three or fewer CL remaining, however, progesterone levels were low (51 +/- 7 ng/ml). It was concluded that ULO can enhance the growth of corpora lutea and perhaps increase their rate of progesterone production.