The effect of a 2-hour intravenous chloroquine infusion (0.015, 0.030 and 1.25 micrograms min-1) on renal fluid and electrolyte handling was investigated in the saline infused, Inactin anaesthetized rat. Blood pressure and glomerular filtration rate were not affected by chloroquine administration, remaining around 128 mmHg and 2.4 ml min-1, respectively throughout the 5-hour post-equilibration period. Chloroquine produced an increase in Na+ and Cl- excretion without affecting the urine flow. By 1 hour after the start of treatment (0.03 micrograms chloroquine min-1) the Na+ excretion rate had increased to 14.5 +/- 2.1 mumol min-1 (n = 6), and was significantly (P < 0.01) greater than in control animals (8.6 +/- 1.0 mumol min-1) at the corresponding time. Parallel but lesser increases in Cl- excretion rates were also observed. The plasma aldosterone and corticosterone levels following either 10, 30 or 120 minutes infusion of chloroquine at 0.03 micrograms min-1 did not differ statistically from each other or from control values. It is concluded that acute chloroquine administration induces an increase in Na+ excretion. The mechanism of this natriuresis cannot be established from the present study, but is likely to involve altered tubular handling of Na+.