Acute renal failure (ARF) due to endotoxins is a common problem in clinical medicine. Endotoxins are released from the outer membrane of the gram-negative bacterial envelope and are composed of lipopolysaccharides (LPS). Although systemic hypotension often is present, LPS-induced ARF is characterized by marked intrarenal vasoconstriction. Both calcium channel blockers and natriuretic peptides are able to antagonize vasoconstricting signals and have been reported to exert beneficial effects in toxic and ischemic ARF: We investigated the effects of diltiazem (Dil, 300 micrograms/kg) or urodilatin (Uro, 40 micrograms/kg) or a combination of both (same doses) on renal function in early LPS-induced ARF: One hour after induction of ARF by i.v. injection of LPS glomerular filtration rate (GFR, clearance of fluorescence-marked inulin) was distinctly reduced to about 54% of basal values. In the following infusion period (60 min) a significant increase of GFR was observed with diltiazem (1.54 +/- 0.11 ml/min), urodilatin (1.60 +/- 0.10 ml/min) and the combination of both drugs (1.66 +/- 0.04 ml/min) compared to controls (1.17 +/- 0.08 ml/min). Combined administration did not cause additive effects. Also 60 and 120 minutes after stopping of drug infusion elevated GFR could be maintained in all experimental groups. Due to their vasorelaxing activity both Uro and Dil induced a decrease of mean arterial blood pressure in comparison with controls and revealed remarkable diuretic and natriuretic activity. In conclusion our results underline that marked intrarenal vasoconstriction in LPS-induced ARF can be antagonized by the well known relaxing potency of Uro and Dil towards vascular smooth muscle and mesangial cells. Both Uro and Dil were capable of improving suppressed renal function in the early phase of LPS-induced ARF in the rat as long as severe systemic hypotension is absent.