The effects of X irradiation and/or hyperthermia on both the intrinsic and total poly(ADPR)synthetase activity in permeabilized human melanoma cells were determined. All treatments were given in the early phase of exponential growth (early log phase). The intrinsic and total enzyme activities were significantly decreased immediately after heat treatment at either 42 degrees or 44 degrees C for 3 hr. These activities remained below the respective control levels over the whole period of culture, and some recovery was observed after 48 hr in both cases. X-ray doses of 3.76 to 11.28 Gy produced an increase in intrinsic activity 30 min postradiation (p.r.) but had no significant effect on the total activity. The increase may thus be associated with DNA repair. At later times after irradiation with 3.76 Gy no significant changes were observed. Combined treatment of X irradiation (3.76 Gy) followed by hyperthermia (42 degrees C for 3 hr) produced an enhanced decrease in intrinsic activity, equivalent to that after hyperthermia at 44 degrees C. The effects on cell proliferation were similar in both cases. However, the total activity was not modified to the same extent after combined treatment. The long-term decrease in total activity at 42 degrees C hyperthermia may be due to inhibitors produced during heat treatment since the enzyme activity was partially restored by changes of fresh medium. These results suggest that heat, X irradiation, and combined treatment act in a different manner on the chromatin-associated enzyme or at different sites on the enzyme complex.