Hepatocyte cultures derived from 15-day foetal rats produce the liver form of pyruvate kinase (EC 126.96.36.199) only after 3 days of culture. The appearance of the liver form of the enzyme can be blocked by the addition of 5-bromodeoxyuridine on day 2 of culture, but not by addition on day 3 of culture. The reversibility of the action of 5-bromodeoxyuridine was shown when the inhibitor was added on day 2 and removed on day 4. By day 6 of culture the liver form of pyruvate kinase was detectable. The specificity of the action of 5-bromodeoxyuridine was monitored by following changes in the closely related embryonic form of the enzyme as a control. This was unaltered by the inhibitor.