The influence of Substance P was studied on the binding characteristics of LH receptors in purified Leydig cells collected from golden hamsters kept under natural long or short days. Substance P exerted a differential effect on the binding capacity of LH receptors. A significant increase in Bmax was estimated in Leydig cells obtained from young hamsters living under long days. In contrast, Substance P reduced the number of the LH binding sites in Leydig cell cultures prepared from adult hamsters housed under short-day conditions.