Ukrain, thiophosphoric acid alkaloid derivatives from Chelidonium majus L. was administered intraperitoneally in a dose of 28 mg/kg (equivalent to 0.1 LD50) every other day for six months to female rats with ovariectomy-induced early osteoporosis. Administration of Ukrain was started on the second day after the surgical operation. At the end of the long-term treatment with Ukrain each rat was tested for the strength of both humeri and some parameters of rat femur were measured. The body weight of ovariectomized rats was also examined. The present results show that the decrease in the mechanical strength of the humeral bones and some changes in the femur caused by ovariectomy were prevented by the six-month treatment with Ukrain. However, in both ovariectomized groups and in ovariectomized rats pretreated with Ukrain an increase of body weight was observed.