The effect of graded doses of diethylcarbamazine on the concentration of microfilariae in the hydrocoele fluid, urine and skin of patients suffering from onchocerciasis was studied. The results showed that a significant number of microfilariae migrated into hydrocoele fluid and urine immediately after the drug was given and thereafter returned to pre-treatment levels rather more quickly in the former than the latter. The increase was found to be roughly directly proportional to the intensity of infection in the skin. By contrast the skin concentration of microfilariae fell sharply after treatment and remained significantly lower than pre-treatment levels for over four weeks. The potential use to which these observations could be put with respect to clinical screening of drugs in onchocerciasis is discussed.