• Twenty-six compounds have been compared with a placebo as to effectiveness in preventing seasickness in 16,920 soldiers and airmen crossing the Atlantic. The most significant benefit was obtained by using 50 gm. of meclizine once or thrice daily, 50 mg. of cyclizine thrice daily, or 25 mg. promethazine thrice daily. Four other drugs were demonstrated for the first time to be effective against seasickness. Earlier reports of the effectiveness of scopolamine hydrobromide were verified as to single doses, but on continued use it gave the most distressing sideeffects of all the prophylactics used. For continued use, meclizine was the most satisfactory. Motion sickness was twice as frequent in those who had had it before as in those with no previous history of it. It was less frequent in the older men, in those who had crossed before, and in those quartered midship.