Uterine activity was measured for a 30 min preinjection period and a 30 min postinjection period for 6 drugs commonly used in labor. Uterine activity was quantitated by measuring the area below the uterine pressure curve with an on-line technique utilizing a voltage control oscillator. Uterine activity readings were obtained every 2.5 min and reported in uterine activity units (UAU). Unmedicated labor demonstrated a relatively static increase in UAU per unit of time in labor. On comparing total UAU before and after injection, only MgSO4 decreased total UAU. Utilizing 10 min summations of UAU, the pre- and postinjection periods were compared to the calculated expected UAU for the 30 min following the preinjection period (assuming no drug was used). For each of the other drugs, the positive slope of the calculated expected UAU shifted to a negative slope of the actual observed UAU following drug administration.