A case is described of a 54-year-old 55 kg patient who presented for clipping of a middle cerebral aneurysm two years after a successful renal allograft. Immunosuppression was maintained with azathioprine 100 mg daily, cyclosporine 300 mg daily and prednisone 10 mg daily. The patient had chronic hypertension controlled with nifedipine 40 mg daily and furosemide 20 mg daily. The cyclosporine level taken on the morning of surgery was 166 micrograms.L-1. Induction of anaesthesia consisted of fentanyl 350 micrograms, thiopentone 125 mg and pancuronium 5.5 mg. Anaesthesia was maintained with nitrous oxide 70 per cent in oxygen and isoflurane 0.5-1.5 per cent. No additional doses of pancuronium were given during the four hour surgical procedure. At the end of surgery, four twitches were present with train-of-four stimulation, but evidence of residual muscle paralysis was present. Residual neuromuscular blockade was reversed with atropine 1.2 mg and neostigmine 2.5 mg. Residual paralysis was present in the Recovery Room and edrophonium 10 mg was given prior to extubation. Clinical testing demonstrated adequate reversal of neuromuscular blockade. Twenty minutes following extubation, increasing respiratory distress was noted. There was clinical evidence of muscle paralysis. The patient was re-intubated. It is proposed that cyclosporine potentiated the pancuronium blockade producing prolonged neuromuscular relaxation resulting in residual paralysis following surgery. The potential interactions of cyclosporine and muscle relaxants deserve further study.