By means of Cresyl violet stainings and immunocytochemical studies on prolactin-, luteinizing hormone (LH)-, and adrenocorticotropin (ACTH)-like immunoreactivities evidence has been obtained that homologous adenohypophyseal transplants survive for many weeks in the rat neostriatum. Gland cells maintain their endocrine properties and appear to secrete prolactin but not ACTH and LH into the surrounding neuropil of the rat neostriatum, since it contains a strong diffuse prolactin-like immunoreactivity. The diffuse zone of prolactin-like immunoreactivity disappears following inhibition of prolactin secretion by the dopamine agonist bromocriptine. The results support the existence of volume transmission in brain and offer a model for its analysis.