Ewes need to be primed with progesterone in order to achieve sensitivity to oestrogen. This relationship was examined in goats. In experiment 1 the median effective dose of oestradiol benzoate (ODB) required to induce oestrus during the breeding season (July) in progesterone-primed ovariectomized does was 15 micrograms, a dose similar to that reported to be required in ewes. In experiment 2 (initiated in March) the requirement for progesterone priming before injection of ODB to induce oestrus after a period without any exogenous hormone treatment, was compared in goats and sheep. Without progesterone priming, ovariectomized does were sensitive to doses of ODB between 20 and 80 micrograms, whereas ewes were not. This indicates that, unlike sheep, goats may not require progesterone priming in order to exhibit oestrus at the beginning of the breeding season. After being primed with progesterone, ewes were more sensitive to ODB than were does. In primed ewes the median effective dose of ODB was 12 micrograms whereas in does it was 27 micrograms. These differences may be due to a seasonal effect on oestrous behaviour or differences in sensitivity to oestrogen between the species.