We have quantified the human chorionic gonadotropin (HCG) and human chorionic somatomammotropin (HCS) serum levels in normal pregnant women at 1-h intervals during 24 h, through radioimmunoassay procedures. The HCG levels were higher during the first trimester, although no circadian variations were detected. Some variations exceeded by ten fold; these variations decreased with advanced periods of gestation. No diurnal fluctuations in serum HCS were apparent; in a case nearly at term, there was an increase from 05:00 h to 09:00 h simulating a circadian pattern, although it was not statistically significant. These data indicate that the placental protein secretion is autonomous and that HCG levels of fluctuation during early stages of pregnancy might be regulated by factors other than the trophoblast.