From Sheiner's equation on pharmacokinetics and pharmacodynamics, we derived a new equation which described the pharmacodynamics of nondepolarizing muscle relaxants during the onset phase. This equation showed that log (l0/l-1) had a linear relation to log (t) where "t" is the time after the administration of nondepolarizing muscle relaxant and "l" and "l0" are the twitch height at t = t and t = 0 respectively. It also implies that the administration dose (D) is inversely proportional to the onset time (OT), i.e., D.OT = const. We proved that these two relations held well for the actual 7 cases of vecuronium use in man. In conclusion, when vecuronium dose level was within 0.15-0.30 mg.kg-1 i.v., the dose was inversely proportional to the onset time which was defined as the time interval from the end of the administration of vecuronium until the single twitch was depressed under 5% of control value, i.e., Dose (mg.kg-1) x onset-time (sec) not equal to 24. During anesthesia with enflurane as well as during neuroleptanesthesia, a dose of vecuronium 0.3 mg.kg-1 (n = 7) was found to produce a duration of neuromuscular blocking action equal to the mean duration produced by pancuronium 0.1 mg.kg-1.