The mechanism by which compensatory ovarian growth occurs is complex and not completely understood. To compare the molecular events in compensatory ovarian growth with those known to occur in other compensatory growth processes such as the regenerating liver, the temporal pattern of proto-oncogene expression and dexoyribonucleic acid synthesis was investigated in rat ovarian tissue after unilateral castration. One hundred fifty female rats were subjected to either a left hemioophorectomy or a sham oophorectomy. Twenty-four rats from each group were put to death at 3 and 14 days after the initial procedure and the ovaries were weighed. There was a mean compensatory weight increase in the right ovaries of the hemioophorectomy group of 7.9% at 3 days and 22.5% at 14 days. The temporal pattern of proto-oncogene expression was determined by removing the right ovary from six rats in each group at 4, 8, 12, 24, 36, and 48 hours after the initial procedure. The ovaries were paired into three samples in each group for each time point and the ribonucleic acid was extracted. Dot blot hybridization was performed on each ribonucleic acid sample with radiolabeled complementary dexoyribonucleic acid probes for the proto-oncogenes c-myc, c-HA-ras, and c-fos. There was no significant increase in proto-oncogene expression in the right ovaries of the hemioophorectomy group when compared with the right ovaries of the sham oophorectomy group. The temporal pattern of dexoyribonucleic acid synthesis was determined by removing the right ovary from three rats in each group at 8, 12, 24, 36, and 48 hours after the initial procedure. Each rat had been injected intraperitoneally with [3H]thymidine 2 hours before the right oophorectomy. The specific activity of dexoyribonucleic acid extracted from each ovarian sample did not demonstrate a significant increase in ovarian dexoyribonucleic acid synthesis after hemioophorectomy or any significant difference in dexoyribonucleic acid synthesis between the hemioophorectomy and the sham oophorectomy groups. This report concludes that compensatory ovarian growth occurs by a mechanism distinct from compensatory growth in the regenerating liver. © 1989.