In premature neonates (25-34 weeks gestation) who were given morphine
intravenously during the first 24 h of life, only morphine, and morphi
ne-3-glucuronide (M3G) were detected in plasma obtained after a 2-hour
loading infusion, but morphine-6-glucuronide (M6G) could also be quan
tified following 24 h of continuous infusion. M3G/morphine and M6G/mor
phine plasma concentration ratios increased significantly with increas
ing birth weight. However, the M6G/M3G plasma concentration ratio decr
eased with increasing birth weight (and gestational age), thus providi
ng the first indication in vivo of the differential development of uri
dinediphosphate glucuronosyltransferases in humans.