The adult greyhound was found to be similar to adult man with respect to kinetic and histomorphometric indices of calcium metabolism. The relationship between trabecular bone tissue balance and the pattern of human PTH fragment 1-34 (hPTH 1-34) administration by daily injections or continuous sc infusions was investigated in this model and the results compared to those from a clinical trial of hPTH 1-34 in involutional osteoporosis (peptide administration by single daily injections). In the dogs, the daily injection regime elevated plasma levels of immunoreactive hPTH 1-34 for no more than 4 h/day. The greyhounds so treated showed significantly increased indices of bone formation (surface osteoid, plasma alkaline phosphatase activity, and skeletal accretion rate of calcium) and resorption (number of osteoclasts, resorption surfaces). Iliac trabecular bone volume increased significantly, as it did in the patients. The infusions did not significantly increase the trabecular bone volume or the 47Ca accretion rate, two parameters which increased in parallel in dogs and patients treated successfully by daily injections. The osteoclastic surfaces, however, were clearly increased by continuous infusions, while the increases in the osteoblastic surfaces were less statistically significant. Since hPTH 1-34 may inhibit osteogenesis in Friedenstein chambers, it is possible that the increased osteoblastic activity induced by the daily injection regime in trabecular bone is dependent on the noncontinuous nature of the PTH stimulus.