SUMMARY Surgical isolation of the hypothalamus from the rest of the brain was used in a study of the hypothalamic control of gonadotrophin secretion. Rats with hypothalamic islands of differing sizes and animals in which the hypothalamus was partially deafferentated were prepared. Oestrous cycles were followed by taking daily vaginal smears and the reproductive state assessed from histological examination of the genital tract collected at autopsy 28 days postoperatively. In rats with 'large' and 'long' hypothalamic islands persistent vaginal oestrus ensued and the ovaries were follicular. Rats in which smaller islands disconnected the suprachiasmatic nuclei from the median eminence displayed persistent vaginal dioestrus and the ovaries contained corpora lutea. Rats with frontal cuts placed anterior to the suprachiasmatic nuclei behaved similarly to rats with 'large' and 'long' islands in developing persistent vaginal oestrus, but when the anterior connexions to the hypothalamus were severed caudal to the suprachiasmatic nuclei the animals had oestrous cycles which were sometimes interrupted by periods of dioestrus of pseudopregnancy length. Posterior and transverse cuts in the hypothalamus did not alter the oestrous cycle although there was some indication that luteal function was favoured.