In cats anaesthetized with i.p. chloralose and pentobarbitone, neuromuscular blockade produced by various doses of rocuronium was measured and dose response curves constructed in the presence of halothane, enflurane, nitrous oxide, propofol, alfentanil, thiopentone, ketamine, diazepam, chlorpromazine, morphine or streptomycin. In general, when a shift in the dose response curve was produced, it was a parallel shift to the left, indicating potentiation. Both halothane and enflurane produced a left shift and a small increase in the time from maximum block to 90% recovery. Nitrous oxide had no effect on the depth of block but delayed recovery. Thiopentone and ketamine potentiated the blocking effect of rocuronium but propofol and alfentanil had no effect. Chlorpromazine and morphine caused potentiation which took 1-1.5 h to develop. Streptomycin had a potentiating effect in four cats but not in two others. Diazepam displaced the dose-response curve to the right in four cats. Prior treatment with suxamethonium had no effect.