Previous studies of contaminant effects on freshwater mussels have been restricted to tissue contaminant concentrations and distribution among tissues. Although no standardized toxicity tests are available, acute toxicity tests have been conducted on freshwater mussels. Chronic toxicity tests which mimic the continuous low-level contaminant exposure these organisms may receive in nature are needed. A physiological approach was taken in this study to examine the sublethal effects of cadmium (0, 30, 100 and 300 ug/l) on the metabolic activity of adult Lampsilis ventricosa. A 28-day toxicity test was conducted using a proportional diluter. Ten mussels were placed into each of 8-57 L aquaria; each treatment was replicated twice. Respiration rate, ammonia excretion rate, clearance rate and food assimilation efficiency were measured on each individual on days 0 (before cadmium exposure), 14 and 28. The tissue condition index ((tissue dry weight/shell dry weight) x 100) of mussels exposed to cadmium for 28 days was compared to the tissue condition index (TCI) in a field sample of 33 L. ventricosa mussels. The O:N ratio (moles oxygen consumed to moles of nitrogen excreted) was estimated from respiration and excretion measurements. Respiration rates in cadmium exposed mussels were significantly (p < 0.05) depressed compared to respiration rates in control mussels. Clearance rates and ammonia excretion rates showed no statistical differences among cadmium treatments, although ammonia excretion rates in mussels exposed to 300 ug/ Cd/l fell from 22 to 5 ug/hr/g dry weight by day 28. By day 28, clearance rates also decreased to one-third of their original value. Assimilation efficiencies increased over the test duration in all treatments. O:N ratios were significantly elevated in mussels exposed to either 100 or 300 ug Cd/l by day 28. TCI values in mussels exposed to cadmium did not vary over treatment levels, but were significantly reduced compared to the field sample;The present research indicated that certain physiological responses can be reliable, sub-lethal indicators of contaminant exposure. However, more basic research on these organisms are needed before they are incorporated into more complex toxicity tests.