A general relation between the rate of onset and rate of recovery from non-depolarizing blockade has been demonstrated, with recovery consistently about ten times slower than onset. This observation has led to the suggestion that non-depolarizing agents share a common mechanism of action. Rocuronium, a recently introduced steroidal non-depolarizing agent, is claimed to have a very rapid onset but an intermediate duration and appears to test this hypothesis. To investigate this paradox we have calculated the rates of onset and recovery of rocuronium using the isolated human forearm and compared them with those of pipecuronium. The mean ratio of recovery time/onset time for rocuronium was 31.3, which is significantly greater than that for pipecuronium, 11.6 (P < 0.01). Whilst pipecuronium conforms to the same general relation between onset and offset described previously for other non-depolarizing agents, rocuronium appears to have a disproportionately rapid rate of onset for its rate of recovery. This suggests that onset, recovery, or both onset and recovery, from rocuronium blockade occur in a different manner to that of other non-depolarizing agents.