In the present investigation, the phenomenon of post-tetanic twitch potentiation (PTP) has been used to provide a sensitive index for neuromuscular blockade during an intense paralysis of the adductor pollicis muscle in man. We have also used and compared PTP assessment with that of twitch, train of four and tetanus in the same muscle and in the absence and presence of vecuronium (50 micrograms.kg-1). Vecuronium had a rapid onset of blockade (5-10s) and an intermediate duration of action (22-26 min). During the onset of blockade, the PTP response was still remaining (residual) whereas all the other mechanical responses disappeared within 2.5 min (Fig. 1b, c, d). Thus, the PTP values increased (upto 300% of control value of 18%, Fig. 1a) with increasing the intensity of neuromuscular blockade. The PTP value provided a better index for assessing the degree of neuromuscular blockade than did the twitch, train of four or the tetanus. However, during the onset of blockade, the PTP technique can also delay the onset of blockade, i.e. it has a decurarizing effect (Fig. 1b, c, d). During recovery of neuromuscular blockade, the PTP is actively involved in the enhancement of spontaneous recovery process, i.e. enhancement of de-curarization with repetitive stimulation of the ulnar nerve. In this respect, the PTP response recovers first, followed by the tetanus, train of four and the twitch responses. Thus the latter may be considered as a more sensitive index for the recovery process.(ABSTRACT TRUNCATED AT 250 WORDS)