Many studies have used adenylate energy charge (AEC) as an index of an organism's metabolic state under conditions of imposed stress, either through natural or xenobiotic stressors. AEC is a linear measure of the ratio of ATP concentration to total adenylate concentration, which ranges in value from 1 in the fully charged state to 0. Paradoxically, high values of AEC are often associated with high toxicant exposures, and low AEC values with low exposures. These discrepancies may be caused by the inability of AEC measurements to adequately evaluate cytosolic adenylate concentrations, which are the critical parameters in enzymatic regulation. Consequently, the goal of this study was to compare AEC values, as measured by high performance liquid chromatography (HPLC), to free adenosine diphosphate (ADPfree) concentrations, as measured using the arginine kinase equilibrium reaction and in vivo 31P-NMR, in red abalone (Haliotis rufescens) in response to either hypoxia or toxicant (pentachlorophenol, or sodium azide) exposure. AEC values remain essentially constant when compared with control animals during periods of stress exposure and recovery. In contrast, calculated ADPfree concentrations are approximately a third of those determined by HPLC and nearly double in response to stress exposure. The physiologic importance of this response is demonstrated by increases in ATP formation via arginine kinase. These results are discussed in light of the pertinent mammalian literature.