Comparative patterns of recovery during successive doses of three new, relatively non-cumulative, intermediate-duration non-depolarizing neuromuscular blocking drugs, BW A444U, atracurium (BW33A) and vecuronium (Org NC 45), were studied in 94 surgical patients during thiopentone in nitrous oxide and oxygen with narcotic anaesthesia. The train-of-four (TOF) pattern of nerve stimulation was used. The spontaneous 5-25% recovery time of the first twitch of TOF showed a statistically significant difference between the initial dose and the fifth incremental dose in the cases of atracurium and vecuronium, but not in the case of BW A444U. The 25-75% recovery times for the final doses were significantly longer than the corresponding times for each drug after the initial doses. The percent TOF ratio at the point of recovery of the first twitch to 95% of the control height was compared for the initial and final doses in a series of doses of each drug. The difference was significant only when the final dose of vecuronium was compared with the initial dose (TOF ratio 79.3 +/- 2.3% v. 64.3 +/- 4.4%; P less than 0.005). Analysis of variance indicates that the TOF ratio at 95% recovery of the first twitch of TOF after the final dose of vecuronium (64.3%) is significantly smaller (P less than 0.001) than that of either BW A444U (78%) or atracurium (84%), indicating that vecuronium appears to show more residual fade and greater cumulative effect after incremental doses than BW A444U or atracurium. The data suggest that the cumulative properties of the new drugs may be ranked as follows: atracurium less than BW A444U less than vecuronium.