The distribution of radioactivity in the rat hypothalamus after intraventricular injection of tritiated luteinizing hormone-releasing hormone (LH-RH) was studied by autoradiography and compared with that expected from Fick's second law which defines the character of ordinary diffusion. The results suggest that LH-RH penetrates through the ependyma and thin subependymal layer in the median eminence and into the suprachiasmatic area by ordinary diffusion. By contrast, concentration of radioactivity in outer layers of the median eminence is increased and, therefore, can not be explained by the diffusion law. In this region incorporation of radioactivity into cell bodies was demonstrated. In the subependymal layer of the median eminence a few rows of silver grains seemingly located in cellular processes were also observed. The diffusion coefficient in the periventricular zone was lower than that in the deeper tissue. It is suggested that the ependymal-subependymal complex of the third and lateral ventricles acts as passive membrane towards LH-RH contained in the cerebrospinal fluid. The significance of this mechanism for the intrahypothalamic transport of RH's is discussed.