Animals and humans undergoing a treatment with cyclosporin (CsA) show a reversible increase in renal vascular resistance and decrease in glomerular filtration rate. The causes of these abnormalities have not yet been established. In animals potential mechanisms for CsA induced renal functional impairment are an increase in urinary thromboxane A2 excretion, plasma renin activity and renal sympathetic nervous system activity and an enhancement of vasopressin stimulated Ca++ mobilisation and cell contraction in vascular smooth muscle cells. In human, the problem is far less clear. CsA induces an inhibition in PRA and urinary prostaglandins excretion. Furthermore CsA does not modify urinary and plasma levels of catecholamines. Whatever the mechanism underlying the vasoconstriction induced by CsA, the inhibition of PGI2 synthesis and angiotensin II formation may participate in the decrease in renal blood flow and glomerular filtration rate which is observed in patients receiving CsA.