Seasonal changes in the content of neurosecretory material in the supra\ x=req-\ optic and paraventricular nuclei and in the pars nervosa of the desert rodents Jaculus jaculus (Oliv.) and Gerbillus gerbillus (Linn.) were followed and compared with those in the white rat kept under laboratory conditions. From the morphological characteristics it was concluded that neurosecretory activity in the desert rodent is minimal in December; signs of increased activity started in March, and maximal activity was reached in June. A decline of neurosecretory activity started in August and continued through the autumn season.In the laboratory white rat, on the other hand, the neurosecretory system showed low activity the whole year round. The quantity of neurosecretory material in the pars nervosa was less in summer than in winter.