The influence of oestradiol-17 beta benzoate, 70 mug administered intramuscularly twice a week for up to four weeks, on collagen synthesis in fractured bones, was studied in young adult castrated female rats. The specific activity of [14C]hydroxyproline in the fracture region of the treated rats was constantly below that of the normal and the castrated control animals throughout the experimental periods. The percentage of incorporated 14C found as [14C]hydroxyproline was also reduced in the fracture regions of the oestrogen treated rats compared to the percentage observed in normal rats. During the first two weeks after fracture and the start of treatment, there was no differences in tensile strength of the fractures and no significant difference in collagen content of the fractured tibia of the 3 experimental groups. The weights of the unfractured tibiae in oestrogen treated rats were lower than those of the control animals. The findings suggest a "slow down" of collagen metabolism after administration of high doses of oestradiol.