The glucocorticoid receptor activity that can be detected in the liver from 15-day foetal rats would appear to be associated with the haemopoietic cells. In hepatocytes, purified by culture for 1-2 days from 15-day foetal rats, the glucocorticoid receptor activity is low and dexamethasone does not induce the enzyme tyrosine aminotransferase. If culture is continued both receptor activity and steroid responsiveness are acquired. Cultured hepatocytes from 19-day foetal liver contain receptor from the first day of culture and, furthermore, the subsequent level of response to glucocorticoids is directly correlated with the actual receptor concentration. It would appear that the glucocorticoid receptor is not acquired by hepatocytes until after 18 days of gestation. Nevertheless, the fact that bromodeoxyuridine has no effect on the rate of accumulation of receptor in hepatocytes suggests that the differentiative event leading to the subsequent appearance of the receptor has already occurred before day 15 of gestation. However, the acquisition of the receptor would appear to be dependent on mitosis as cytosine arabinoside can inhibit the process.