The effects of 3 consecutive days of oral chloroquine (1 mg/100 g body weight) on kidney function and blood pressure were studied in male Sprague—Dawley rats that were challenged with hypotonic saline infusion 24 h after the last chloroquine administration. The rats were anesthetized with Inactin [5 ethyl-5-(1′-methylpropyl)-2-thiobarbiturate, Byk Gulden] and continuously infused with 0.077 M NaCl for 8 h; urine flow and electrolyte excretion rates were monitored during the last 5 h. Blood pressure and glomerular filtration rates were also measured. Kidney function was compromised in chloroquine-treated rats, which retained significantly more of the infused Na+and Cl−by comparison to control-vehicle-treated rats. Throughout the experimental period, chloroquine-treated rats exhibited low blood pressure (80 mm Hg vs. 127 mm Hg) which was associated with low glomerular filtration rate. The plasma aldosterone concentrations were significantly (p<0.01) elevated in rats pretreated with chloroquine at the end of the 8-h infusion of hypotonic saline, but corticosterone levels were significantly (p<0.01) lower in the treated rats. It is concluded that chloroquine administration impairs kidney function, resulting in inappropriate Na+and Cl−retention. This effect is likely to be mediated via chloroquine-induced increases in plasma aldoster-one concentration and lowering of GFR.