Simultaneous milk and saliva samples were collected over a seven-day period from five lactating subjects after they had ingested two tablets of chloroquine sulfate (300 mg base). The samples were analyzed for chloroquine content by use of a combination of thin-layer chromatography and spectrophotometry. Regression analysis of two variables subject to error was used to determine the milk:saliva levels relationship. The Student's t test for paired data at .05 level of significance was used to evaluate the relationship between chloroquine levels in milk and saliva. The milk:saliva chloroquine concentrations ratio obtained by regression analysis was 0.89 +/- 0.08 (r = .93; P less than .05), and this compared favorably with the value of 0.99 +/- 0.07 obtained from the ratio of the AUC. There were no significant differences in the Tmax, Cmax, AUC, clearance, and elimination half-life values of chloroquine in milk and saliva (P greater than .05). Milk and saliva appear to be part of the central compartment. The amount of chloroquine estimated to be consumed by a nursling over a 24-hour period is about 0.55% of a 300-mg dose consumed by the mother. The data suggest that milk chloroquine levels can be estimated from saliva levels.