Studies were conducted to evaluate the efficacy of scopolamine, absorbed through intact skin, in preventing motion sickness at sea. Efficacy of transdermal scopolamine was compared with oral dimenhydrinate and placebo. Transdermal applications were made 4 to 16 hr before exposure to motion. Dimenhydrinate or placebo was given 1.5 hr before motion and again 2.5 hr after motion began. Comparison with placebo indicated that transdermal scopolamine provided protection against motion sickness at a significance level of p = 0.0001 and oral dimenhydrinate at a level of p = 0.05. Dry mouth, drowsiness, and blurred vision associated with transdermal scopolamine therapy were minimal.