Plasma concentrations of adenosine 3', 5' monophosphate (cyclic AMP), free fatty acids (FFA), glycerol and glucose were measured after 24, 48 and 72 h fasting in healthy male students. There was a marked rise in plasma concentrations of cyclic AMP, FFA and glycerol from 24 to 48 h fasting with no further changes from 48 to 72 h fasting. Plasma glucose concentrations fell. After 72 h fasting, inhibition of adipose tissue lipolysis with a nicotinic acid analogue effected no significant change in plasma cyclic AMP concentrations, indicating that the elevated plasma nucleotide concentrations during fasting were not related to stimulation of lipolysis.