Morphological data are reported of 69 rats with prolonged vaginal oestrus following the placement of a basal lesion in the anterior hypothalamus, and of 137 rats with regular cycles which were grouped according to the stage of the vaginal smear cycle (early dioestrus, late dioestrus, pro-oestrus, oestrus). The groups of rats with regular cycles comprised 99 intact rats and 38 rats with hypothalamic lesions. Pituitary and uterine weights fluctuate during the cycle, with highest values at pro-oestrus and oestrus. In rats with prolonged vaginal oestrus, the weight of the uterus equals that of regularly cycling rats in oestrus, whilst the weight of the pituitary gland markedly exceeds the highest value found during the normal cycle. Some of the implications of these findings are discussed. Subsequent papers will contain data on pituitary and serum contents of ICSH and FSH in the various groups of control rats and in rats with prolonged vaginal oestrus.