Physiological correlates of the ontogenetic transition from generally nonflying hive bees to foragers were described. Peak foraging activity of workers occurred between 15 and 32 days of age in six honeybee colonies, depending on season. Whole wet body mass decreased ~40% in correlation with the transition to foraging behavior. The entire loss of mass occurred in the abdomens, with 85% of the total decrease occurring in the digestive tract exclusive of the honey crop. Maximal thorax-specific rates of oxygen consumption ($\dot{V}O_{2}$) were 10% higher.in foragers than in 8-12-day-old hive bees. The increase in thorax-specific maximal $\dot{V}O_{2}$ combined with the decrease in body mass yielded a 50% increase in mass-specific maximal $\dot{V}O_{2}$ in foragers compared to hive bees. Glycogen stores of the thorax doubled in foragers. Pyruvate kinase activities increased fivefold up to 4 days of age and thereafter remained constant. Citrate synthase activities increased tenfold up to 4 days of age and then...