The purpose of this study was to evaluate the dose-effect relationship of recombinant human PTH [hPTH-(1-84)] and synthetic human PTH [sPTH-(1-34)] for the skeleton of estrogen-deplete osteopenic rats. Ex vivo densitometry of regionalized whole femurs and histomorphometry of proximal tibial cancellous bone were the end points. Retired breeder female rats, aged 6-7 months, were used. Ten were killed at baseline, and the rest were ovariectomized (OVX). On day 42, a pre-PTH treatment OVX group was killed. The rest were then treated by daily sc injection with hPTH (0, 1.55, 15.5, or 155 micrograms/kg BW.day) or sPTH (0.55, 5.5, or 55 micrograms/kg BW.day) and killed on day 70. The level of cancellous bone mineral was lower in pretreatment OVX rats than at baseline. It was higher in rats treated with 15.5-155 micrograms/kg.day hPTH and 5.5-55 micrograms/kg.day sPTH than in pretreatment and vehicle-treated OVX rats. Cancellous bone volume was also lower both 42 and 70 days after OVX. Although hPTH did not affect cancellous bone volume, treatment with 5.5-55 micrograms/kg.day sPTH caused higher bone volume than in either pretreatment or vehicle-treated OVX rats. Trabecular number declined after OVX and did not change with PTH treatment. In contrast, trabecular thickness declined after OVX, but was higher after 15.5-155 micrograms/kg.day hPTH and 5.5-55 micrograms/kg.day sPTH treatment. In OVX rats, the amount of mineralizing surface was greater by day 42 and fell toward control levels by day 70. It was greater in rats treated with 15.5-155 micrograms/kg.day hPTH or 5.5-55 micrograms/kg.day sPTH than in vehicle-treated OVX rats. On a molar basis, sPTH was modestly more potent than hPTH. Osteoclast surface was not affected by PTH treatment. Treating estrogen-deplete osteopenic adult rats for 28 days with 15.5-155 micrograms/kg.day hPTH or 5.5-55 micrograms/kg.day sPTH increases trabecular thickness, but not trabecular number, to cause a rise in bone mass. The extent of mineralizing surface rises without a change in resorption surface. The marked rise in mineralizing surfaces suggests that extended in vivo treatment with PTH activates osteogenic precursor cells near once quiescent surfaces to become osteoblasts.