Pregnant women (336) participated in a study to determine the possible relation between maternal plasmatic zinc levels (PZL) and uterine contractibility. PZL during gestation and at the moment of delivery were evaluated in relation to the mode of delivery, the duration of the active period of delivery, and the presence of hemorrhage during the first 30 min postpartum. A significant difference (P < 0.001) between PZL at different periods of gestation was found. There were significant differences (P < 0.05) between PZL and duration of the active period of delivery and PZL and final mode of delivery. In 10 cases, a cesarean section had to be performed due to uterine atony. In these patients, the PZL were significantly lower than in the rest. The role of Zn in uterine muscle contractibility is discussed. The possibility of supplementation in those women with low Zn levels (< 60 .mu.g/dl; 9 .mu.mol/l) is recommended.