1. The acute effects of chlorothiazide and bendroflumethiazide on renal Li+ clearance (CLi) were studied in Na+-restricted healthy humans during maximum water diuresis. 2. Chlorothiazide, which has marked carbonic anhydrase inhibiting activity, increased CLi by about 25%. The concomitant rise in uric acid clearance, maximum urine flow and bicarbonate excretion suggests that this drug suppressed proximal reabsorption through carbonic anhydrase inhibition, which would also explain the observed fall in glomerular filtration rate (increased glomerulotubular feedback activity). 3. Bendroflumethiazide, which lacks carbonic anhydrase inhibiting activity, did not affect CLi or any of the other above-mentioned variables. 4. It is concluded from the lack of an effect of bendroflumethiazide on CLi that Li+ is not reabsorbed in thiazide-sensitive segments of the human distal nephron. The rise in CLi after chlorothiazide is most likely due to suppressed Li+ reabsorption in the proximal tubules resulting from carbonic anhydrase inhibition. 5. The results of this study are compatible with the concept that CLi is an index of Na+ and water delivery from the proximal tubules in humans.