The crop sac response was determined in adult pigeons injected with either chicken hypothalamic extract (H.E.) or cortical extract for periods of up to 3 weeks. H.E.-injected pigeons showed substantial crop sac stimulation. The fine structure of adenohypophyseal prolactin cells was markedly different in the two groups, the H.E.-injected pigeons showing a strong resemblance to “lactating” control pigeons. The apparent discrepancy between the level of the crop sac response and the degree of activity in the pituitary prolactin cells is discussed.