Three experiments demonstrated that chronically administered restraint inhibits the development of DMBA-induced rat mammary tumors. The effect is exhibited in a lower proportion of positive responses, increased latency of tumor development, and lower number of tumors in positive animals when restrained animals are compared with controls. Organ weights failed to show a stress response to restraint. Molar activity data from Experiment III indicated that early in the experiment activity increased among the restrained animals on release from restraint. A fourth experiment, which employed a crystal accelerometer to assess activity, produced a similar activity pattern and also provided evidence of adrenal ascorbic acid depletion as a function of restraint.