BACKGROUND: Proteinuria and hyperhomocysteinaemia are independently associated with increased risk of atherosclerosis and cardiovascular disease. The available data on plasma homocysteine (Hcy) level in patients with nephrotic syndrome (NS) are contradictory with increased, decreased and unchanged values reported by different investigators. The majority of Hcy in the plasma is bound to albumin whose urinary losses and diminished plasma concentration are the defining features of NS. The present study was designed to explore the effect of NS on plasma concentration and urinary excretion of Hcy and hepatic expression of methylenetetrahydrofolate reductase (MTHFR) and cystathionine-ß-synthase (CBS), the key enzymes in re-methylation and trans-sulphuration of Hcy, respectively. METHODS: Sprague-Dawley rats were rendered nephrotic by IP injection of puromycin aminonucleoside. Urine and plasma were used for measurement of Hcy, and the liver was processed for assessment of MTHFR and CBS protein expression. RESULTS: Compared with the controls, nephrotic rats showed heavy proteinuria, hypoalbuminaemia, hypercholesterolaemia, normal plasma creatinine and creatinine clearance, reduced plasma Hcy, increased urinary Hcy, and downregulation of CBS but not MTHFR expression. Plasma Hcy correlated directly with plasma albumin and inversely with urinary protein excretion. The urinary Hcy excretion correlated directly with urine protein excretion. CONCLUSIONS: NS results in significant reduction in plasma total Hcy concentration which is due to the reduction in albumin-bound Hcy as opposed to the free Hcy fraction. This is coupled with increased urinary excretion of albumin-bound Hcy. In addition, NS results in down-regulation of CBS which can curtail conversion of Hcy to cysteine and reduce production of H(2)S which is an important endogenous signalling molecule.