The aim of the study was to investigate the interaction of the psychosomatic stress and the Ehrlich ascites tumor (EAT) growth in mice. The stressor consisted of combination of a light signal followed by a mild electric shock. The first experiment was performed on CBA mice irritated for 0, 2 and 4 weeks respectively, prior to intraperitoneal transplantation of the EAT. In the second study, mice of BALBc strain were used. Stress was applied 4 weeks before the tumor transplantation and continued throught the experiment. Both the irritated and the nonirritated animals were subjected to either intraperitoneal or subcutaneous inoculation of the EAT. In both experiments, mice were left to live until their spontaneous death. In the first experiment, after a 2-week irritation the experimental animals showed a significantly longer survival time as compared to the controls. Longer or shorter duration of the irritation had no significant effect on the results obtained. Results yielded by the second experiment showed no significant difference in the time of survival of the irritated and nonirritated animals after the i. p. transplantation, whereas after the s. c. inoculation of the EAT, the irritation significantly increased the survival period. The EAT in irritated mice was observed to have invaded the vitals later and less frequently than in the nonirritated animals. Quantitative histological analysis of some endocrine and lymphatic organs revealed signs of stress in the experimental animals. The EAT transplant, per se, had a stressogenic effect too.