A summary is presented of a programme of work investigating the comparative efficacy of the two drugs most commonly used for seasickness prophylaxis in the Royal Navy, hyoscine and cinnarizine. The programme had both laboratory and sea-trial components. It was shown that hyoscine was a more effective drug than cinnarizine, but cinnarizine had less marked side effects. This comparative superior tolerability of cinnarizine decreased as motion sickness increased. Guidance is given as to the optimum indications for each drug, together with prophylactic regimens.