The effect of 48 h of fasting in C57B1/6J-ob/ob and +/+ mice on body weight (BW), blood glucose (BG), serum immunreactive insulin (IRI), plasma immunoreactive glucagon (IRG) and on tissue levels of cyclic adenosine monophosphate (cAMP) were studied. Both groups of mice lost weight and demonstrated a decrease in BG and IRI with fasting. However, the BG and IRI of the ob/ob animals were initially highter and remained higher than those of the 2% of their initial weight while the +/+ lost 14 %. The +/+ mice exhibited an increase in cAMP levels in skeletal muscle, fat and liver with fasting, while the ob/ob mice had increased levels of cAMP in fat, but not in muscle. They also had a paradoxical decrease in liver cAMP levels with fasting, and associated with this was the lack of stimulation of glycogenolysis. Glycogenolysis was significant in the livers of fasted +/+ mice. The plasma IRG levels of the fed ob/ob mice were significantly higher (1.8) times) than those of the fed +/+ mice. Islet cAMP levels were decreased with fasting in ob/ob mice. However, the levels were significantly higher in 48-h faster ob/ob mice compared to the fasted +/+ group. The apparent paradoxical response to fasting observed in the livers of the ob/ob mice remains unexplained.