Adenine nucleotide and glycogen concentrations were measured concurrently in white muscle of mummichogs Fundulus heteroclitus after the fish were exposed to stressors that either caused an increase in energy use (metabolic loading) or damaged metabolic function (toxic inhibition). When fish were exposed 4 h to 1% unbleached kraft mill effluent (which causes metabolic loading) in the presence of 6 mg/L dissolved oxygen, glycogen and AMP concentrations significantly (P ≤ 0.05) decreased below control values, whereas ATP, ADP, and total adenylate (TA) concentrations as well as the adenylate energy charge (AEC = [ATP + 1/2ADP]/TA) were unchanged. When dissolved oxygen was below 1 mg/L, the effluent caused significant decreases in glycogen, ATP, and TA, but not in ADP, AMP, or the AEC. The combined effect of effluent and hypoxia caused more significant drops in ATP or TA pool. When fish were exposed to 60 μg/L DDT (a toxic inhibitor) for 4 h, none of the measured energy variables changed even though this concentration was lethal after several days. At a concentration of 100 μg/L DDT, all variables except ADP decreased significantly from control values, which may have reflected energy depletion of the muscle in response to nerve spasms rather than a direct toxic effect on the muscle itself.