The anabolic effect of intermittent treatment with parathyroid hormone (PTH) on cortical bone was investigated. Groups of rats were injected with human PTH (1-34) or PTH (1-84), 1.1, 3.3, 10, and 30 nmol/kg/day for 30 days. A dose-related increase in bone formation rate at the femoral middiaphysis was found at both the periosteum and the endosteum and also an increase in bone mass, with no change in the bone lengths or body weight gain of the rats. The highest mineral apposition rate, as analyzed by tetracycline labeling, was found at the periosteal postero-medial aspect and at the endosteal anterior aspect. This pattern of bone modeling was also found in the PTH-treated animals, although more and more areas were included in bone mineral apposition. The PTH treatments did not change the porosity of the cortical bone nor the concentration and biochemical stability of the collagen. The highest doses of PTH resulted in a slight reduction in the ash concentration of cortical bone. No differences were found between the effects of PTH (1-34) and PTH (1-84) on bone formation rate, bone mass, porosity, and biochemical parameters. Consequently, intermittent treatment with PTH increased the formation of cortical bone dose dependently, at both the periosteum and the endosteum and increased the bone mass of these growing rats, with no change in the body weight gain or femoral growth rate compared with the control animals. The responses of the cortical bone modeling were increased by the PTH treatments without changing its direction or pattern.