This study evaluated the potency and time course of action of vecuronium in patients with primary hyperparathyroidism (HPT) and marked hypercalcaemia during nitrous oxide-opioid anaesthesia. Twenty ASA physical status I and II patients were studied by measuring the force of contraction of the adductor pollicis in response to stimulation of the ulnar nerve: ten control patients and ten patients with HPT and ionized calcium concentration over 2.80 mEq.L-1. After induction of anaesthesia with thiopentone and maintenance with N2O/O2 and fentanyl, vecuronium was administered to determine cumulative dose-response curves. When maximum block had been obtained, twitch height was maintained at 10% of baseline value over 20 min by adjusting the infusion rate of a syringe-pump containing vecuronium and vecuronium plasma concentration (EC90ss) was determined. During spontaneous recovery, after termination of infusion, the recovery index, the time from 25 to 75% recovery, was measured. The dose to produce 90% block was greater in the HPT than in control group: 69 (24) vs 54 (18) micrograms.kg-1 (P < 0.02). The calculated ED50 was also greater in HPT: 42 (4) vs 31 (5) micrograms.kg-1 in controls (P < 0.001). (Values are given as mean and coefficient of variation). The slope of the dose-response curve, the dose necessary to maintain 90% block, and the EC90ss did not differ. The RI25-75 was slower in the HPT group although the difference did not reach statistical significance. It is concluded that hyperparathyroidism with hypercalcaemia increases vecuronium requirement; only during the onset of neuromuscular blockade.