On Day 0, young adult female F344 rats were adrenalectomized and intrasplenically implanted with a pituitary gland and capsule containing estrone. All were thereafter given 2.5 mg deoxycorticosterone per week and the choice of saline or tap water. This treatment yields high prolactin levels and glucocorticoid deficiency (Prl+/Glc-). On 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 on Day -1 yielded tumor latencies and frequencies insignificantly different from unirradiated controls and significantly different from the dose on Day +48. Total carcinomas per rat day at risk were better fit 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 was thus influenced by total clonogen numbers and/or proliferation rates; that of neutrons was not.