Human placental lactogen (HPL) levels were estimated before and during labor in normal pregnant subjects. There was no change in the circulating levels of HPL in the 14 days preceding the onset of labor. During labor, there was a small, progressive fall which may reflect uteroplacental ischemia associated with uterine contractions. No relation was found between HPL levels in the last weeks of pregnancy and the gestation at which spontaneous labor occurred. It is not possible to attribute any direct role to HPL in the initiation and maintenance of labor.