Tamm-Horsfall protein (THP), a normal constituent of mammalian urine, has been determined in rat urine under various conditions in an attempt to elucidate the physiological role of this glycoprotein. Experiments were designed to assess whether THP production is related to the process of urine concentration or to the transport activity of the thick ascending limb of the loop of Henle (TAL), the nephron segment where it is produced. For this purpose, THP excretion was measured, by radioimmunoassay, in adult male rats under 4 different conditions induced by the following chronic treatments: (1) furosemide (12 mg/day in osmotic minipumps); (2) increased water intake; (3) antidiuretic hormone (ADH) infusion (50 ng DDAVP/day in osmotic minipumps) in rats of the Brattleboro strain with hereditary hypothalamic diabetes insipidus; (4) high-protein (32% casein) versus low-protein diet (10% casein). Each experiment included 6 experimental and 6 control rats. After treatment for 1-3 weeks, 24-h urines were collected for determination of urine flow rate, osmolality, and creatinine and THP concentrations. No significant changes in THP excretion were observed in experiments (1) and (2) despite 5- to 7-fold-differences in urine flow rate. Antidiuretic hormone treatment in (3) slightly lowered THP excretion (287 +/- 53 vs. 367 +/- 41 micrograms/day per 100 g body weight; p less than 0.005), whereas high-protein diet, in experiment (4), led to a 50% increase in THP excretion (446 +/- 57 vs. 304 +/- 79 micrograms/day per 100 g body weight; p less than 0.001). Expressing THP excretion relative to that of creatine did not change these findings. These results show (1) that chronically established changes in the level of diuresis, chronic furosemide-induced blockade of the Na,K,Cl-cotransporter or the absence of ADH in Brattleboro rats have little or no impact on the level of THP production, and (2) that THP production is independent of the intensity of transport in the TAL, since two conditions which both are known to increase the transport rate of solutes in the TAL (ADH infusion and high-protein diet), resulted in opposite changes in THP excretion. It is concluded that the rate of THP synthesis is neither linked to the process of urine concentration nor to the ion transport activity of the TAL.