SUMMARY The influence of age and sex on the cortisol content of adrenals, liver, kidney, lung, heart, spleen, muscle, cerebrum, brain stem and blood was studied in white guinea-pigs. In prepuberal animals no significant difference between sexes was found in the cortisol content of different tissues. In adult female animals, the blood cortisol level was significantly higher than in the male animals. Significantly more cortisol was found in kidneys of adult male than of female animals. In adult guinea-pigs, the cortisol content was lower in every tissue examined, except liver, compared with young animals. These differences were highly significant. In female guinea-pigs the decrease in tissue cortisol content with age was more pronounced than in male animals.