The experiments were designed to investigate the possible role of ovarian hormones in causing increased synthesis by neurones of the supraoptic (SO) and paraventricular (PV) nuclei previously found during pregnancy and lactation. Changes in the dry mass of nucleoli of these neurones were measured to indicate their synthetic activity. When a normal suckling stimulus was applied only after a delay of 7 days post-partum the nucleolar changes in PV and SO neurones that followed were similar to those that occur normally when a litter is suckled from birth; these changes thus do not depend on the hormonal state of late pregnancy or early lactation. After unilateral nipple line ablation prior to pregnancy nucleolar dry mass of PV and SO neurones increased bilaterally in a subsequent lactation. These results do not distinguish bilateral distribution of the milk ejection pathway from the action of hormones on PV and SO neurones. Nucleoli of SO and PV neurones in virgin rats were larger after treatment with a large dose of oestradiol for 5 days, but were not affected by ovariectomy, progesterone treatment or by the stage of the oestrous cycle. It is concluded that increased synthetic activity in SO and PV neurones during lactation depends upon the suckling stimulus rather than on changes in secretion rates of ovarian hormones; oestradiol may cause the nucleolar changes in PV and SO neurones seen at the end of pregnancy.