The nature of the room temperature density–dose relationships in polytetrafluoro-ethylene (PTFE) have been studied over the dose range of 104 to 109 rads. Under various conditions, samples of PTFE were irradiated in combinations of neutron and gamma (reactor and 60Co) radiation fields resulting in density increases up to approximately 5%. For the dose range 105 to 108 rads, the increase in density measured at 0°C. is approximately proportional to the logarithm of the radiation dose. For doses less than about 105 rads, the increase in density appears to be directly proportional to the dose, while above 108 rads the density reaches a maximum and then begins to decrease. By examining the changes in slope and displacement of the curves which occur under various conditions, several effects are discernible. Compared to the effects of γ-rays alone, the density increase per dose in the 105 to 108 rad region is lower for cases where both neutrons and γ-rays are present in the radiation field. This may suggest that competing processes are involved in producing density changes. The discernible difference between effects of γ-radiation and reactor radiation appears to be unique in polymers. The ambient temperature during irradiation and the post-irradiation heat treatment of the specimens affect the changes in density. For dose rates over the range 0.1 × 106 to 13 × 106 rads/hr., no dose rate effects have thus far been observed. The feasibility of using PTFE for some types of radiation dosimetry has been demonstrated, and its possible usefulness is being investigated.