【Abstract】 Objective To observe the effects of calcitriol on intracellular calcium concentration ([Ca2+]i) and hypertrophy of cardiomyocytes induced by parathyroid hormone (PTH). Materials and Methods Cultured neonatal rat cardiomyocytes were loaded with Fluo-3/AM, and [Ca2+]i was determined by a laser confocal microscope. Hypertrophic response of neonatal rat cardiomyocytes was assayed by measuring cell surface area and protein content. Results (a) When the extracellular Ca2+ was 2.5mmol/L, rapid stimulation of PTH1-34 at the concentration of 0.1 and 1μmol/L significantly elevated fluorescence intensity of [Ca2+]i in cardiomyocytes (P＜0.01). However, when the extracellular fluid contained no calcium or the cardiomyocytes were pre-treated with 10μmol/L nifedipine, 1μmol/L PTH1-34 did not induce the increase of fluorescence intensity any more. (b) When the cardiomyocytes were treated in 0.01 and 0.1μmol/L PTH1-34 for 7 days, their surface area, protein content and [Ca2+]i increased significantly, especially those treated in 0.1μmol/L PTH1-34. If the cells were co-treated with 0.001μmol/L calcitriol and 0.1μmol.L-1 PTH1-34, their surface area, protein content and [Ca2+]i attenuated significantly. However, when the calcitriol concentration increased to 0.1μmol/L in the co-treatment experiment, these changes remained. Conclusion Cardiomyocyte hypertrophy was induced by PTH1-34, and was inhibited by appropriate concentration of calcitriol. Calcium influx through voltage-dependent channel induced by PTH may be one of the important mechanisms leading to cardiomyocyte hypertrophy.