The phytocomponent p-hydroxycinnamic acid (HCA) has been shown to have a stimulatory effect on bone formation and an inhibitory effect on bone resorption in rat femoral tissues in vitro. The preventive effect of HCA on bone loss induced in streptozotocin (STZ)-diabetic rats was investigated in vivo. Rats received a single subcutaneous administration of STZ (6.0 mg/100 g body weight), and then the animals were orally administered HCA (0.25, 0.5, or 1.0 mg/100 g body weight) once daily for 14 days. STZ administration caused a significant decrease in body weight and a significant increase in serum glucose, triglyceride, and calcium levels, indicating a diabetic state. These alterations were significantly prevented by administration of HCA (0.25, 0.5, or 1.0 mg/100 g). Calcium content in the femoral-diaphyseal and -metaphyseal tissues was significantly decreased in STZ-diabetic rats. This decrease was significantly prevented after administration of HCA (0.25, 0.5, or 1.0 mg/100 g). Alkaline phosphatase activity in the diaphyseal and metaphyseal tissues was significantly decreased in STZ-diabetic rats. The decrease in diaphyseal alkaline phosphatase activity in STZ-diabetic rats was significantly prevented after administration of HCA (0.5 and 1.0 mg/l00 g). The diaphyseal DNA content was also significantly decreased in STZ-diabetic rats. Administration of HCA (0.25, 0.5, or 1.0 mg/100 g) caused a significant increase in DNA content in the diaphyseal and metaphyseal tissues in STZ-diabetic rats. This study demonstrates that the intake of HCA has preventive effects on bone loss in STZ-diabetic rats, and that the intake has partially restorative effects on serum biochemical findings in the diabetic state.