The vascular disturbance associated with ciclosporin (CS) nephrotoxicity is poorly defined in both the normal and transplanted human kidney. Six bone marrow transplant recipients were studied before, during and after administration of CS. By analysis of time activity curves and clearance of Tc-99m DTPA, renal blood flow (RBF), filtration fraction (FF) and GFR were shown to fall on CS (GFR: p less than 0.001; RBF, FF: p less than 0.01). These haemodynamic variables did not fully recover with dose reduction or after discontinuation of therapy. Plasma renin activity (PRA) aldosterone and atrial natriuretic peptide did not change although there appeared a tendency for PRA to fall on CS. There was a reversible normokalemic metabolic acidosis. CS levels stayed within the therapeutic range. The partial reversibility of GFR, RBF and FF is consistent with CS-induced functional disturbance and reflects sensitivity to microvascular and tubular injury of the normal human kidney.