1 In previous studies on the renal targeting of the ACE inhibitor captopril, we demonstrated that a 6 fold increased concentration of this drug could be obtained in the kidney after conjugation to the low-molecular-weight protein lysozyme. In this study, we investigated in unrestrained rats whether systemic administration of captopril-lysozyme also results in an enhanced effect on renal parameters, relative to the systemic effects. 2 Renal effects: intravenous infusion of captopril-lysozyme for 6 h resulted in a more pronounced increment of renal blood flow (31+/-2% vs 17+/-4% at 0.5 mg kg(-1) 6h(-1), P<0.01) and an approximately 5 fold enhanced natriuresis (167+/-17% vs 36+/-7% at 1 mg kg(-1) 6 h(-1), P<0.001) in comparison with equimolar amounts of captopril as a free drug. In correspondence with these findings, renal ACE inhibition was potentiated approximately 5 fold (-50+/-4% vs -22+/-3% at 1 mg kg(-1) 6 h(-1), P<0.001). 3 Systemic effects: conjugated captopril did not affect blood pressure in dosages up to 5 mg kg(-1) 6 h(-1). This effect coincided with a less pronounced inhibition of the pressor response to intravenously administered angiotensin I (-12+/-3% vs -66+/-5% at 1 mg kg(-1) 6 h(-1), P<0.001), and a markedly attenuated plasma ACE inhibition (-19+/-2% vs -37+/-3% at 1 mg kg(-1) 6 h(-1), P<0.001) compared to an equivalent dose of free captopril. 4 An experiment of continued intravenous administration of captopril-lysozyme for 7 days in nephrotic syndrome demonstrated that the conjugate is also active in renal disease: the antiproteinuric response was substantially augmented (-67+/-5% vs -15+/-7% at 4 mg kg(-1) 24 h(-1), P<0.001) compared to the free drug, in the absence of blood pressure reduction. 5 These data demonstrate that intravenous administration of a captopril-lysozyme conjugate leads to more selective renal ACE inhibition and enhanced renal effects as well as less systemic effects compared to captopril itself.