The bioavailability of an aqueous solution of chlorothiazide and three commercially available chlorothiazide tablets was assessed in adult mongrel dogs. In two crossover urinary excretion studies, six fasting dogs received single 500-mg doses of chlorothiazide as an aqueous solution, one 500-mg originator tablet on two separate occasions (Tablets A-1 and A-2), two 250-mg originator tablets (Tablet B), or one 500-mg generic tablet (Tablet C). 6-Amino-4-chlorobenzene-1,3-disulfonamide )chloraminophenamide), a pharmacologically active hydrolysis product of chlorothiazide was not detected in any urine sample. Urinary recoveries of chlorothiazide after oral administration, expressed as the mean (range) percent of the dose, was only 22.0 (8.41-33.9), 15.7 (10.2-25.0), 20.7 (7.25-31.0), 18.5 (8.72-33.2), and 21.9% (6.69-41.1%) for the aqueous solution and Tablets A-1, A-2, B, and C, respectively. Considerable interindividual variation and some intraindividual variation were observed. No statistically significant difference in bioavailability existed among the aqueous solution and Tablets A-2 and B, between Tablets A-1 and C, and between Tablets A-1 and A-2.