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The aim of this work was to determine whether Psammomys obesus living in its natural habitat displays variation in the function of the endocrine pancreas. Live animals were collected regularly through the year by strip sampling. Plasma glucose varied during the year. Glucose levels were higher in periods I and III (autumn and spring) than in periods II and IV (winter and summer). Pancreatic insulin contents were higher in periods I and II than in periods III and IV. Plasma insulin also varied with the season in synchrony with glucose variations except during period IV, in which elevated levels of insulin were associated with low levels of glucose. The high insulin levels observed in period IV were resistant to the effects of food restriction in that period and were probably related to the presence of the greater levels of insulin in period III (spring). The dry season (period IV) corresponding to the period of restricted diet appears necessary to restore elevated levels of insulin to baseline. Morphometric quantitation of the insulin, glucagon, and somatostatin positive islet cells did not reveal seasonal variation. However, the volume density of A and D cells in P. obesus reached high values of 32 and 15%, respectively. The volume density of insulin cells amounted to about 53%. It is concluded that P. obesus living in its natural habitat is a nondiabetic healthy gerbil adapted to desert subsistence by endocrine feedback, imposed by the seasonal variation in food availability. (C) 1998 Academic Press.