| Abstract: | 1 Plasma naloxone levels were determined by RIA over a period of 6--36 h in three groups of neonates, (1) those given 35 microgram i.v. (n = 6), (2) those given 70 microgram i.v. (n = 6) and (3) those given 200 microgram i.m. (n = 17) naloxone HCl within 1 min of birth. 2 After intravenous administration of 35 and 70 microgram of naloxone peak levels of 4--15 ng/ml and 9--20 ng/ml respectively were reached in 5--40 min and the mean plasma half-life after both doses was 3.1 +/- 0.5 h. 3 Peak levels of 7--35 ng/ml were reached 0.5 to 2 h after intramuscular administration of 200 microgram. The fall in concentration after this was consistently biphasic with the levels declining rapidly between one and four hours and then slowly from four hours onwards. 4 Plasma levels at 24--36 h after i.m. administration were as high as they were 4 h after i.v. administration of 35 microgram and this may account for the prolonged duration of action when this route is used. |
