The pathophysiology and prevention of contrast media (CM)-induced nephropathy in chronic renal failure (CRF) are still ill defined. GFR, RPF, endothelin-1 (ET-1) levels, urinary sodium concentration, and fractional excretion of sodium were measured in CRF patients undergoing water diuresis in basal conditions and 20 to 120 min after an iv bolus of either the high-osmolar CM diatrizoate (D) or the low-osmolar CM iopamidol (I). The two CM induced an immediate and progressive decline of both GFR and RPF in the absence of hypovolemia, more pronounced in D (-36% at 120 min) than after I (-19% at 120 min; P < 0.05 versus D). Both CM determined a marked and steady increase of the fractional excretion of sodium. The natriuresis could not be totally ascribed to a CM-induced osmotic diuresis as because the urinary sodium concentration markedly increased. In two further groups of patients receiving D, we studied the effects of pretreatment with a single dose of either captopril or nifedipine. Both drugs, although not preventing the increase of natriuresis, partially antagonized D-induced renal hypoperfusion: GFR and RPF were equally reduced by 20% in D/captopril and D/nifedipine (P < 0.05 versus D). In unpretreated patients receiving either D or I, plasma ET-1 did not change but urinary levels increased; these changes were, however, dissociated from those observed in renal hemodynamics. Both plasma and urinary levels of ET-1 did not vary in pretreated groups. The 72-h follow-up evidenced a significant reduction of renal function only in the unpretreated D group. Therefore, the main findings after CM administration in CRF patients are: (1) an immediate GFR decline proportional to the osmolarity of CM and secondary to the renal hypoperfusion that is neither caused by hypovolemia nor mediated by ET-1, (2) an early tubular dysfunction at the level of the proximal nephron, and (3) a protective effect of single-dose pretreatment with either captopril or nifedipine on D-induced acute and short-term GFR changes.