In order Co estimate the human placenta! lactogen (HPL) level and its value as an indicator of fetoplacental function during labor, we determined HPL levels (N=225) before, during, and after labor in normal (N=16) and preeclamptic (N=14) subjects or in patients with benign intrahepatic cholestasis of pregnancy (N=5). During labor, greater decreases in this value were found in preeclamptic than in normal subjects and similarly in mothers with fetoplacental dysfunction than with normal fetoplacental function. The rupture of the membranes had no effect on the level of HPL, which was not related to parity, oxytocin infusion, time interval from rupture of the membranes to delivery, nor to relative placental weight. The half-life of HPL varied in the range of 20-23 minutes immediately after deliver) and in the range of 30-39 minutes some time later. During labor, greater decreases in HPL level in cases of preeclampsia or fetoplacental dysfunction may be caused by relative uteroplacental ischemia during uterine contractions, but from this finding it is hard lo expect any advantage of HPL as a monitor of fetoplacental function during labor.