The effects of ciclosporin on active Heymann's nephritis starting at different times after induction was studied. When given at the time of antigen injection, ciclosporin blocked both free antibody and circulating immune complex formation. Immunopathology and renal function tests remained normal in this group despite a rise in both these parameters to control values after ciclosporin was discontinued. When given at a later stage of the disease process, despite suppression of both antibody and complex formation, neither pathology nor functional parameters were modified. This suggests either a limited time frame for immunologically mediated injury in Heymann's nephritis or that early treatment with ciclosporin may permanently alter a vital primary step in the immunopathogenesis.