Female rats of the Sprague-Dawley strain were mated 60 days after a single intravenous injection with 5 mg of 7,12-dimethylbenz(a)anthracene. In order to maintain the lactation for more than 3 weeks, 5 pups (5 to 10 days old) were attached to each mother rat throughout the experiment. Experimental rats were divided into 6 groups: 1. non-lactation (4 rats); 2. 3-week lactation (8 rats); 3. 6-week lactation (10 rats); 4. 8-week lactation (7 rats); 5. 12-week lactation (9 rats); and 6. 17-week lactation (4 rats). Lactating condition was judged from the body weight curves of the 5 pups. The appearance, number and diameter of tumors were recorded every day. The average of the tumor induction time was 140+/-74 days. The tumors were divided into 3 types: 1. progressive; 2. static; and 3. regressive. During pregnancy few tumors of the regressive type were noted. However, tumors of the progressive type increased significantly with the weeks of pregnancy. In the first week postpartum, although the total number of tumors was approximately the same as in the last week of pregnancy, the tumors of the progressive type decreased significantly and the tumors of the regressive type increased. In this period most tumors showed a tendency to regress. In the non-lactating group, the tumors of the progressive type began to increase from the fifth to seventh week postpartum without relation to the lactating period (3 to 17 weeks). Vaginal smears were examined every day in all mother rats to determine the recurrence of the estrous cycle. The first estrus appeared 7 days on an average after delivery in the non-lactating group; 27 days on an average in the 3-week lactation group; and 35 to 38 days on an average in the 6- to 17-week lactation groups. Under the condition of high prolactin level, the behavior of the tumors of the progressive type seemed to be related to estrogen. This was also supported by histological findings in the ovaries.