To determine whether present methods of international transport of essential drugs by sea adversely affect their quality. Controlled longitudinal study of drug shipments sent by sea from Unicef in Copenhagen to Lagos; to Mombasa and by land to Kampala; and to Bangkok. 11 essential drugs were stored in four locations on board the ships. Main shipping routes from Unicef, Copenhagen, to tropical countries. Temperature and relative humidity in the test packs during the journey. Amount of active ingredient in the drugs before and after shipment. Temperatures recorded within the test packs range from -3.5 degrees C to 42.4 degrees C and were 3-12 degrees C higher than the ambient temperature. Relative humidity within the packs ranged from 20% to 88%. Differences between the locations on board were negligible. Ergometrine injection, methylergometrine injection, and retinol capsules lost 1.5-5.8% of their activity. Ampoules of ergometrine showed a large variation in the amount of active ingredient after shipment, with three of 80 samples having concentrations 60% below those stated. Ampicillin, benzylpenicillin, phenoxymethylpenicillin, and tetracycline were not affected by transport. Drugs were exposed to a much higher temperature and humidity than is recommended by the manufacturer, especially in tropical harbours and during inland transport. Except for ergometrine and methylergometrine the transport would not affect clinical effectiveness.