Cultured rat hepatocytes were used to study the effects of hormones on the production of apo A-I. In addition, we compared these effects with the production of albumin. Hepatocytes were isolated from normal adult rat livers and cultured in MEM, as nearly confluent monolayers. In the absence of hormones, apo A-I and albumin accumulated in the culture medium almost linearly for periods up to 24 h. The rates of accumulation of apo A-I and albumin in the medium were 22 ng/mg cell protein per h and 1.2 micrograms/mg cell protein per h, respectively. During the incubations the cellular contents of apo A-I remained constant. Insulin stimulated the production of albumin at concentrations over 10(-10) M, but inhibited the production of apo A-I at concentrations over 10(-8) M. Dexamethasone showed no significant effects on albumin production but stimulated apo A-I production at concentrations over 10(-6) M. Glucagon inhibited the production of albumin and apo A-I dose-dependently at concentrations over 10(-10) M. Thus, the production of albumin and apo A-I are presumably controlled by different regulatory mechanisms.