Differences in binding and structural properties of ovine testicular FSH and LH receptors were investigated. The ovine FSH receptor did not discriminate between FSH of different species, although equine FSH was more reactive. In the same tissue, however, the LH receptor showed marked preference for ovine and bovine LH, reacting very weakly with other preparations of pituitary LH. Human chorionic gonadotrophin also reacted partly with the ovine LH receptor at 25 °C. However, at 4 °C. the optimum temperature for binding of the LH receptor to its homologous hormone, the receptor displayed no recognition for chorionic gonadotrophin preparations. Affinity cross-linking studies with ovine testicular membrane suggested that the ovine FSH receptor has an M r of 70 000, which is very similar to that observed in the porcine ovary. The M r of the ovine LH receptor was estimated to be 150 000, which is different from those of other mammalian species, including those that have been cloned. The data suggest that the binding and structural properties of the ovine FSH receptor are similar to those of other mammalian FSH receptors, whereas the ovine LH receptor appears to differ from other mammalian LH receptors in having a different M r and in being more stringent in its requirement for pituitary LH.