The rate of osteogenesis was studied in 8 non-pregnant, non-lactating Friesian dairy sheep, 3-6 years old, by means of a treatment with 3 different bone seeking agents. Four sheep were fed a low calcium ration (LCa:1.8 g Ca/d) and four other sheep a high calcium ration (HCa:12.7 g Ca/d). The bone markers, oxytetracycline-HCl, alizarine-complexion and demeclocycline-HCl, were administered at intervals of 6 weeks, and the sheep were killed 1 week after administration of the last marker. In undecalcified cross sections from the middle of ribs 2, 10 and 12, and from the proximal and distal parts of rib 10, the numbers of labelled osteons and the number of osteons with 1, 2 or 3 markers were counted under fluorescent microscopy. In the ribs of sheep from the LCa group, the number of labelled osteons and the quantity of labels per osteon tended to be higher than those of sheep from the HCa group. When osteogenic activity was compared in the different sites of ribs analysed, lowest osteogenic activity was observed in the proximal part of the 10th rib. The use of fluorescing markers offers the possibility of studying osteogenic activity over a certain period of time in adult sheep.