Effects of Radiation on Living Organisms Early and Late Effects of Radiation of Different Project Title: Quality and at Different Dose Rate 16. Technical Progress in FY 1973: (Cont'd.) RR-03-O1-(a3) 0. 30-0,43-MeV rather than the desired 0.43-MeV. The experiment was terminated at 60 days after exposure. Even at this short interval, 16 rads produced an incidence of 13%; 4 rads, 7% and 1 rad, 2%, while 240 R of x-ray produced 13% and 80 R was without detectable effect. These results suggested a rather large RBE, hence, another experiment was begun using 0.43-MeV neutrons in che doses of 0125, 0.5, 2 or 8 rad and x-ray doses of 30, 60 or 90 R. At some eight months after beginning the experiment, the cumulative percent of rats with mammary tumors after 30 R is matched almost exactly by the 0.5 rad dose of neutrons and 0,125 rad has produced more mammary neoplasms than no exposure. Again, a rather high RBE is suggested by these experiments that are still in progress, To study the possible sparing effect of lowering the dose-rae on rat mammary carcinogenesis, Co-60 gamma radiation was given at dose-rate of 0.03 R per minute at doses of either 88, 265, 530, or 795 R and at a dose-rate of 10 R per minute at doses of either 88 or 265 R. When the experiment was terminated, 340 days after beginning the exposures, the incidence of rats with mammary neoplasia or mammary fibroadenomas was not different at the two dose rates at total doses of either 88 or 265 R, The incidence of rats with mammary adenocarcinomas was not different at 88 R, although there was a somewhat smaller response at 265 R with the lower dose-rate. The dose sparing effect of a lower dose-rate was modest at best under the conditions of this experiment. It is well known that the Long-Evans strain of rat exhibits a much smaller mammary neoplastic response to either x radiation or dimethylbenz- (a)anthracene (DMBA) than does the Sprague-Dawley strain of rat. To see if the low response of the Long-Evans rat was due to failure of DMBA to reach the target tissue, DMBA was applied directly to mammary tissue in vitro, Preliminary results indicate that Sprague-Dawley rats still show a larger mammary neoplastic response than do Long-Evans rats even after mammary tissue from both strains received directly the same amount of DMBA. These results are interpreted to mean that the strain difference in response to carcinogens does not obtain at the level of mammary tissuecarcinogen interaction. Rats thymectomized on the day of birth were studied in regard to their mammary neoplastic response to DMBA, but the results were inconclusive due @ poor rate of complete thymectomy. In studying the rate of cell tation and/or synthetic activity in regard to mammary neoplasia, in or drugs that increase prolactin secretion were used. Rats while lactating have shown essentially the same neoplastic response Mammary adenofibromas are absent in Lewis rats as non-lactating sister rats. while Sprague-Dawley rats, similarly treated with x rays or DMBA, have reached a 50+% incidence of rats with mammary adenofibromas, (See Continuation Sheet) b1192b2 (Shellabarger) RX- 169