The present study was to investigate the mRNA, protein expression and the activity of calcineurin in the hypertrophic heart, and to determine the effect of calcineurin inhibitor--cyclosporine A (CsA) on the regression of cardiac hypertrophy in renovascular hypertensive rats. Renovascular hypertension was induced by two kidney-one clip methods. Two months after the operation, cardiac hypertrophy was determined by histological analysis performed in some rats (2K1C-2M), then the rats were subdivided into 2 groups: (1) 3-month old two kidney-one clip group (2K1C-3M) with rats receiving 0.9% NaCl per day for one month, and (2) CsA-treated group with rats treated with CsA for one month. Sham-operated rats were used as control. The ratio of the left ventricular weight to tibial length (LVW/TL), the area of cardiac myocyte, mRNA and protein expression and the activity of calcineurin were determined. Both the LVW/TL and the cardiomyocyte area were significantly larger in 2K1C-2M and 2K1C-3M rats than in age-matched sham-operated rats. Treatment with CsA significantly attenuated the increase in the LVW/TL as well as the cardiomyocyte area. The mRNA, protein expression and the activity of calcineurin were significantly higher in 2K1C-2M and 2K1C-3M rats than those in the age-matched sham-operated rats, while the elevation of mRNA, protein expression and activity of calcineurin were significantly suppressed in the CsA-treated rats. In conclusion, calcineurin plays a role in the progression of cardiac hypertrophy in renovascular hypertensive rats. The inhibition of calcineurin can reverse cardiac hypertrophy.