The effects of environmental factors, particularly moisture content and storage treatment, on responses to gamma irradiation were studied in seeds of Pinus rigida Mill. The variable germination results obtained with commercially purchased seed indicated a need for close control of conditions before and during experiments. Techniques were developed for uniform collection and treatment of seeds from trees growing in the natural pitch pine-oak forest at the Brookhaven National Laboratory, Upton, New York. To study the effect of exposure rate and post-irradiation treatment on germination, samples of seeds were exposed to ten total exposures (0–48 kr) at two exposure periods (16 and 80 hr). After irradiation, seeds were either sown immediately or stored for four weeks at 4 °G under dry or moist conditions. Moist stratification caused an increase in germination that was paralleled by a decrease in cytological damage at all exposure levels, but there was no difference between the exposure periods within any post-irradiation treatment. The nature of recovery after irradiation was studied using a 16-hr exposure period, five total exposures (0–24 kr) and six stratification periods (0–6 weeks). Stratification decreased the radiation effect, and the recovery period at lower levels of radiation was shorter than at higher levels. Stratification increased germination at all radiation levels but not the rate of seedling development. Samples were equilibrated for eight days at six moisture contents (2·4–16·1%) and exposed for 16 hr to five exposures (4·36 kr). Increased moisture content up to 8·9% decreased the radiosensitivity, and between 10·9 and 16·1%, LD 60 values for germination were similar (20–21 kr). In this range there was no loss of germination capacity up to 12 kr, although seedling development was retarded. Recovery increased with increasing stratification period and moisture content. Using a 16-hr exposure period and a total exposure of 5 kr, there were significant differences in seed germination between ten trees and within individual trees. Seeds from the upper crown were generally more sensitive to gamma rays than those from lower positions. The variation in radiosensitivity was not related to differences in seed size. Throughout the paper, the LD 50 values that are reported deal with germination percentages.