Approximately 3% of the adult population have a genetic basis for decreased PChE enzyme activity. Levels of PChE activity in healthy term newborns and infants up to 4 months of age have been reported to range from 22-86% of normal adult values. The degree of decreased enzyme activity in premature infants is unknown. Levels of PChE were measured in 39 healthy premature infants (single determinations in 12 and serial weekly determinations in 27).Birth weight ranged from 1430-2070g and GA ranged from 28-37 wks. Ten adult donor and 20 term newborn blood samples were analyzed for comparison purposes. Levels of PChE were measured in duplicate by a kit dye reduction method (normal adult reference value 7-19U/ml). Abnormally low PChE levels were found in 10% of adult, 15% of term newborn and 20% of premature infant blood samples(NS). This incidence of low PChE activity is 3-5 times that reported for adults using a different method. In 6 premature infants with initially low PChE levels, the enzyme activity rose to normal adult range within 2 weeks. In 2 other premature infants PChE levels remained low and most likely represented congenital PChE deficiency. There was no difference in PChE activity on the basis of sex, race or GA. Hospitalized premature infants exhibit similar rates of quantitative PChE deficiency when compared to adults and term infants and are at potential risk for significant complications when exposed to depolarizing muscle relaxant agents. When succinylcholine use is considered in premature infants, measurement of PChE levels or use of alternative drugs may be indicated on the basis of transient PChE deficiency.