Following the hormonal treatment of rats with high prolactin levels and glucocorticoid deficiency (Prl+/Glc-) for 48 days (Day +48), total recoverable mammary DNA was increased by more than sevenfold, tritiated thymidine uptake by nearly fourfold, and total mammary clonogens by about fivefold. Irradiation with 4, 40, and 80 cGy X-rays on Day +48 increased total mammary carcinomas per rat-day-at-risk linearly with dose, and 40 and 80 cGy significantly decreased first carcinoma latency. A dose of 40 cGy X-rays before hormone treatment (Day -1) yielded tumor latencies and frequencies insignificantly different from unirradiated controls but significantly different from those when the dose was given on Day +48. Total carcinomas per rat-day-at-risk were fitted better by a function of dose to the power 0.4 than by a linear function after exposure to 1, 10. and 20 cGy fission neutrons, and 10 and 20 cGy significantly shortened the time to appearance of the first cancer. In contrast to results with X-rays, 10 cGy neutrons on Day -1 yielded tumor frequencies and latencies insignificantly different from 10 cGy neutrons on Day +48. The carcinogenic action of X-rays, but not of neutrons, was thus influenced by total clonogen numbers and/or proliferation rates.