To investigate the time relationships involved in cyclosporin-induced nephrotoxicity we studied changes in blood pressure, renal haemodynamics and sodium excretion in 22 adult patients with insulin-dependent diabetes mellitus treated with cyclosporin (CsA) for 4 +/- 2 days, compared to 22 insulin-dependent diabetic patients receiving conventional insulin therapy, who were matched for age and duration of diabetes. To further clarify the pathogenic role of the renin-angiotensin system, insulin-dependent diabetic patients receiving CsA were studied before and after sublingual administration of 75 mg captopril. An average of 4 days CsA treatment markedly increased blood pressure and renal vascular resistance, but did not alter glomerular filtration rate, renal plasma flow, sodium urinary excretion, or body-weight. The marked renal vasoconstriction without early changes in GFR suggests that the late decrease in GFR may involve other factors in addition to renal hypoperfusion. Acute inhibition of angiotension II formation was still able to decrease blood pressure and renal vascular resistance, although not to normal control values. These results indicate that a physiological concentration of angiotensin II may potentialise but may not be the sole factor involved in the vasopressor effect of CsA.