The effects of cyclosporine (CY) were determined to see whether CY has the potency of reducing urinary protein excretions due to puromycin aminonucleoside (PAN) in nephrotic rats which are the model of minimal change nephrotic syndrome (MCNS). CY had a marked reduction of urine protein excretion during treatment with this drug, but it was observed that an increment in urinary protein excretion was introduced within a few days after the withdrawal of CY. Immunological and histological studies were performed to know the mechanisms of reducing urinary protein excretion in PAN rats. Firstly, the lymphocyte subsets of peripheral blood were evaluated using monoclonal antibodies of W3/25 and MRCOX/8. We found that the W3/25/MRCOX-8 ratio increased to 3.2 +/- 0.3 in the PAN rats but significantly decreased to 2.8 +/- 0.3 with CY treatment. From these data it was impossible to decide whether the effect with CY had direct activity or not, because the interleukin-2 (IL-2) activity was not measured in rats. Secondly, polyethyleneimine (PEI) staining was performed to detect the sites of anion charge on glomerular basement membrane (GBM), and anionic dots on GBM in the CY-treated rats were clearly defined as nearly normal. Although there persist questions as to why nephrotic rats relapsed after the withdrawal of CY treatment, our results indicate that CY might reduce urinary protein excretion through immunological and non-immunological mechanisms.