The effectiveness of an antihistamine, chlorpheniramine, in suppressing chloroquine-related pruritus has been investigated in 38 patients, employing two modes of therapy. In the first treatment mode, chloroquine and chlorpheniramine were given concomitantly to the patient, and in the second treatment mode, chlorpheniramine was given 3 h after chloroquine ingestion. The extent of suppression of the pruritus with the first treatment mode was 40% but with the second mode it was 70%. The onset of itching was always delayed usually by 6-10 h. Bringing the time of antihistamine therapy closer to the onset for the reaction therefore, increased the extent to which pruritus was suppressed.