The relation of plasma concentration of d-tubocurarine (dTc) to neuromuscular blockade, and the distribution and urinary excretion of dTc was determined in neonates (n = 4), infants (n = 6), children (n = 8), and adults (n = 8). The plasma concentration-time course curves to 24 hr are best described for all groups by three-compartment models. Both neonates and infants exhibit decreased plasma clearance (CLP), 1.1 +/- 0.08 and 1.0 +/- 0.06 ml X kg-1 X min-1, and in addition a prolonged t1/2 terminal phase, 311 +/- 44 and 306 +/- 35 (mean +/- SEM, min). The neonates' 24-hr urinary excretion, 27 +/- 2 (mean +/- SEM, % total dose) is significantly less than the adult value, 45 +/- 4% total dose. There was no significant difference seen in the log plasma concentration-evoked compound electromyogram (ECEMG) response between 20-80% paralysis for adults, children, infants, and five of the seven neonates studied. Two of the neonates had a significant shift of their log concentration-response curve to the right. There was also no significant difference between any of the groups in the time for 50% return of ECEMG stimulus height or the time required for recovery of the ECEMG from 25 to 75% of control value. for recovery of the ECEMG from 25 to 75% of control value.