1. Dopamine-beta-hydroxylase (D beta H) activity was measured in sera from ten healthy male students after 2 and 3 days fasting which is associated with increased circulating noradrenaline. 2. Circulating D beta H remained essentially unchanged throughout the fasting period, although 'stress parameters' like plasma glycerol and free fatty acid concentrations were elevated by a factor of about two from first to second and third day of fasting (P less than 0-01). 3. Serum D beta H activity is not a sensitive index of increased sympathoadrenal activity in fasting man.