The effects of hypothalamic deafferentations and brain lesions on adrenocorticotropin (ACTH) and corticosterone (CS) responses after neural stimuli were studied in rats. It was found that the various modalities are mediated by different neural pathways. Photic and acoustic stimuli utilize a posterior input, whereas the sciatic and olfactory modalities are mediated by anterior afferents and forebrain limbic structures. The effects of limbic extrahypothalamic structures on the hypothalamus are transmitted by both anterior and posterior inputs. The medial forebrain bundle (MFB) was found to have a differential role in the mediation of ACTH and CS responses. To identify the neurotransmitters involved, rats were injected with 6-hydroxydopamine in the MFB and 5,7-dihydroxytryptamine in the raphe nuclei. The depletion of hypothalamic norepinephrine and serotonin caused inhibition of the adrenocortical responses only after photic stimulation, and only of responses to limbic stimuli in the former group. In view of the location of corticotropin-releasing factor in the paraventricular nucleus, responses to afferent stimuli were studied in this region in identified neurons projecting to the median eminence. Stimuli that increased or decreased CS secretion facilitated or inhibited cell firing, respectively, thus demonstrating a possible electrophysiological correlate of neuroendocrine regulation.