We determined net photosynthesis and respiration rates for sago pondweed (Potamogeton pectinatus) exposed to various concentrations of 11 herbicides widely used in Maryland during the past decade. Net photosynthesis and respiration were determined by measuring changes in the oxygen content of solutions containing dilutions of technical grade herbicides. At 20–22° C and 58 μmol/m2/sec of photosynthetically active radiation (PAR), oxygen production of undosed plants averaged 0.72–2.03 mg/g fresh wt/h. Respiration rates of undosed plants averaged 0.46–0.60 mg O2/g fresh wt/h. Nominal herbicide concentrations (ng/L) that reduced net photosynthesis by 50 percent (IC50) were: metribuzin, 8; atrazine, 29; cyanazine, 32; linuron, 70; simazine, 164; and paraquat, 240. IC50 values for 2,4-D, acifluorfen, glyphosate and metolachlor exceeded the maximum test concentration of 10,000 ng/L. The IC50 value for alachlor was estimated to be between 1,000 and 10,000 ng/L. None of the herbicides tested had a significant effect on dark respiration.