Diurnal changes in content of cytoplasmic RNA in the supraoptic nucleus (SON) and arcuate nucleus (AN) neurons of the mouse hypothalamus induced by ACTH were analysed microspectrophotometrically. Single injections of ACTH were administered in a dose of 100 I.U./kg at the same or different times of day and night. A single dose of ACTH administered at the same or a different time caused, within the first 4 h, a decrease in the total content of cytoplasmic RNA in neurocytes of the examined hypothalamic nuclei, and subsequently, a significant increase in later hours as compared to control values. At the same time, ACTH induced a significant shift in maximum and minimum values of cytoplasmic RNA level in SON and AN neurocytes, as compared with control material.