The pattern of apolipoprotein (apo) A-I, A-IV and E expression in developing rat liver was established by determining steady-state levels of the respective mRNAs. Apo A-I and A-IV altered in a coordinate fashion; the transcripts were detected from day 13 of gestation, whereas apo E was first detected on day 19 of gestation. Apo A-I and A-IV mRNA levels increased with developmental age until day 19, then declined until birth, after which they increased. In contrast, apo E mRNA levels progressively increased from day-13 gestation until 3 days postnatal at which time it reached adult levels. In cultured hepatocytes established from immature (15-day gestation) and near-term (19-day gestation) fetuses the difference in regulation between apo A-I and A-IV and apo E was also observed. In 3-day-old fetal hepatocyte cultures established from 19-day gestation rats, dexamethasone, insulin, thyroxine and glucagon each substantially increased levels of apo A-I and A-IV mRNA but markedly decreased apo E mRNA. Thus fetal and adult hepatocytes respond similarly to the hormones tested with respect to apolipoprotein expression. Unexpectedly, 15-day gestation hepatocytes expressed apo E in culture, even without hormone supplementation. The discrepancy between in vivo and in vitro data suggests that, in the fetus, apo E expression may be suppressed by high levels of circulating steroid, insulin and thyroxine and that establishment of the hepatocytes in culture removes the inhibition, thereby inducing apo E expression in these immature cells. The data are also consistent with the view that the same group of hormones may be responsible for regulating levels of apo A-I and A-IV in the perinatal period. Both apolipoproteins progressively increase as the fetus reaches term at a time when these hormones which induce their expression are also increasing.