A continuous-flow respirometer was constructed to measure the effect of pesticides and related pollutants on oxygen consumption of estuarine fishes. The parts of the respirometer in contact with pollutants were constructed of glass and teflon for efficiency in cleaning. Filtered, irradiated sea water of constant temperature and salinity was gravity-fed through ten experimental and ten control respiration chambers in which individual fish were held. Flow rates through the chambers were controlled by stopcocks and measured by flowmeters; dissolved oxygen was determined by the Winkler method before and after water passed through each chamber. Pollutants were metered into the experimental chambers by syringe pump.