1. Electrophysiological experiments have been performed on intact cycling female rats to investigate the neural connexions that exist between the medial forebrain bundle, the anterior hypothalamic region, which included the preoptic area, and the basal hypothalamus. Recordings have been made from a total of 351 neurones in the anterior hypothalamus of which 216 were responsive to stimulation of either or both the medial forebrain bundle and basal hypothalamus (arcuate and ventromedial nuclei).2. Forty-six of these cells were responsive to a stimulus applied both to the medial forebrain bundle and the basal hypothalamus with a variety of response combinations. The majority of neurones were orthodromically activated by stimulation in both sites. Inhibition by stimulation of the medial forebrain bundle coupled with orthodromic excitation from the basal hypothalamus, or the reverse situation, was also encountered frequently.3. A few cells were antidromically invaded by the stimulation of the medial forebrain bundle and these received orthodromic or inhibitory inputs from the basal hypothalamus, although one unit outside the anterior hypothalamus was antidromically activated by both stimuli.4. Ninety per cent of all the doubly responsive units that could be antidromically activated by stimulation of the basal hypothalamus received an orthodromic input from the medial forebrain bundle, and no cells in the anterior hypothalamus that projected to the basal hypothalamus were found to receive an inhibitory input from the medial forebrain bundle.5. These results provide electrophysiological evidence for inhibitory and excitatory inputs from the medial forebrain bundle to the preoptic and anterior hypothalamic cells that either project to, or receive connexions from, the basal hypothalamus. Neurones in the preoptic area which project to the basal hypothalamus are implicated in the control of anterior pituitary function, particularly gonadotrophin secretion. These experiments, coupled with functional studies, suggest that there is an excitatory input from the medial forebrain bundle to these preoptic and anterior hypothalamic cells which may modulate adenohypophyseal secretions.