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Objective This study aimed to determine the effect of parathyroid hormone-related protein (PTHrP) on proliferation of cardiac fibroblasts (CFs) in primary cultures of neonatal Wistar rats. Methods Different PTHrP concentrations were added to CFs of neonatal Wistar rats and the cells were grouped according to the concentrations added. A verapamil (VPL) group and a calcitriol (CAL) group were also established. Changes in cell proliferation and in cyclic adenosine monophosphate and calcium ion levels were identified and recorded. Results We found that as the concentration of PTHrP increased, 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT, a tetrazolium salt) colorimetric absorbance values (A values) decreased. These values in the PTHrP groups were significantly lower than those in the control group. MTT colorimetric A values and 3 H-thymidine deoxyribose intake were lower in the VPL group, low-dose CAL group, and the PTHrP 10 −7 mol/L group compared with the control group. However, MTT colorimetric A values and 3 H-thymidine deoxyribose intake were higher in the high-dose CAL group than in the PTHrP 10 −7 mol/L group. As PTHrP concentrations increased, intracellular cyclic adenosine monophosphate concentrations also increased. Conclusion PTHrp, VPL, and low-dose CAL inhibit proliferation of CFs, while high-dose CAL promotes proliferation of CFs.