Thein vitroeffect of prolactin and hydrocortisone, singly or in combination, on the DMBA‐induced mammary tumour in the rat was investigated by organ cultures. Prolactin had a stimulating effect on the DNA synthesis in one out of five tumours. Hydrocortisone induced lobulo‐alveolar differentiation but reduced the DNA synthesis or failed to affect it. The addition of prolactin to hydrocortisone did not alter this response. Virus induced rat sarcomas were studied as control tissue. Prolactin inhibited growth of one sarcoma. Hydrocortisone stimulated growth of sarcomas. The relevance of these findings in relation to similarin vitroexperiments is discus