The passage of chloroquine into semen was investigated in four healthy men following the oral administration of four chloroquine sulphate tablets (600 mg base) to each subject. Chloroquine was found to be excreted into semen with a slow transfer rate constant of 0.0002 min-1, and the semen/plasma ratio based on regression analysis was 0.40 +/- 0.06 (mean +/- s.d.). It is concluded that the passage of chloroquine from plasma to semen occurs by passive diffusion.