Selected physiological variables of rock bass <i>Ambloplites rupestris</i> collected by electroshocking, and then confined in on-site cages for one week, were examined at different times of the year. This was done in both an area contaminated with mercury, and an upstream reference site on the South River, Virginia. When compared to the upstream fish, rock bass from the mercury contaminated area had significantly higher concentrations of the metal in both the muscle and liver. On the whole, significant physiological differences between the two groups of rock bass were rather limited, although sharp differences in liver ascorbic acid and glutathione were observed between those fish which were collected in July. The July rock bass also had higher levels of liver mercury than that observed in fish collected at other times. Most seasonal changes were associated with the start of the spawning time (May), and included sex related differences in plasma calcium, liver glycogen, and ascorbic acid which were not observed on any other sampling date. Female rock bass were found to have higher liver glutathione concentrations than males throughout most of the study. Rock bass from both sites were also exposed to 150 ppb mercuric chloride for 96 hours in the laboratory. These results suggest that mercury exposure may cause a depression in the liver glutathione of these fish, although no changes in liver ascorbic acid were observed. Physiological measures on the rock bass maintained in the laboratory indicated this treatment was more stressful than confinement in cages in the river. / M.S.