The dependence of somatotropin-binding activity of theca and granulosa tissues of hen preovulatory follicles on pH of incubation buffer was studied. A rise of [125I] somatotropin specific binding to theca tissue with decrease of pH from 6.3 to 5.0 was found to be due predominantly to an increase of the number of somatotropin-binding sites rather than to their affinity. At the same time, the character of change of the level of the hormone specific binding to granulosa cells (at pH 6.0) and to theca crude membrane fraction (at pH 6.7) in the process of development of preovulatory follicles was similar with that revealed at determination of concentration of somatotropin receptors in these tissues at pH 5.0. The obtained data indicate that the majority of somatotropic receptors in hen follicular tissues is latent, at least for in vitro binding, and can be detected at low pH. Nonetheless, the relative changes of somatotropin-binding activity of granulosa and theca tissues in dynamics of folliculogenesis do not depend on pH of incubation medium.