Noninbred Sprague-Dawley rats bearing N-methyl-N-nitrosourea (MNU)-induced mammary tumors were ovariectomized at 2.5, 3.5, and 4.5 months after the first MNU injection to determine the response to castration as a function of the time of tumor appearance. Tumor number and tumor size recorded at weekly intervals revealed that the tumors in the control rats continued to grow during each of the three observation periods, but that tumor growth was significantly less during the third period. Ovariectomy performed at 2.5 months after the first MNU injection produced a stabilization of tumor number; when performed at 3.5 or 4.5 months, it resulted in a slight decline in tumor number. Although tumor size decreased slightly in rats that were ovariectomized at 2.5 months, many of the tumors regrew during the last 2 weeks of observation. This was not true, however, for rats that were ovariectomized at either 3.5 or 4.5 months after the first MNU injection. The level of receptors for 17 beta-estradiol (E2), progesterone, and prolactin were significantly reduced by ovariectomy. E2 receptors, which ranged from 2.04 +/- 18 to 2.24 +/- 0.24 (mean +/- SEM) pmol/g tissue for the first and second groups of control rats, declined to 0.93 +/- 0.14 pmol/g tissue for the ovariectomized rats at the end of the last interval studied (5.5 mo after the first MNU injection). This study suggests that hormone responsiveness (response to ovariectomy) of MNU-induced mammary tumors increases slightly with the age or time of appearance of the tumors.