In 30 healthy obstetrical patients, determinations of serum cholinesterase were performed during labor, one day and three days postpartum. Ten patients were studied late in pregnancy and 6 weeks postpartum. Compared to nonpregnant values as determined 6 weeks postpartum, there was a 27.7 per cent reduction of enzyme activity during late pregnancy, a 15.6 per cent decrease during labor, a 24.6 per cent decrease one day postpartum, and a 32.3 per cent decrease two days later. Compared to values during labor, over 90 per cent had falls of cholinesterase activity three days postpartum.Low cholinesterase values (below 55 units per ml.) were found in 10 per cent of patients in late pregnancy and during labor, in 20 per cent of patients one day postpartum, and in 60 per cent of patients two days later.A case is presented of prolonged paralysis following the use of 400 mg. succinylcholine during cesarean section. Subsequent studies disclosed abnormally low serum cholinesterase activity, but a qualitatively normal enzyme.