The accurate determination of respiration rates is critical to any study of energy flow through marine ecosystems, and in energy budgets of individual marine invertebrates, respiration generally accounts for the major portion of the organic matter ingested. However, there are limitations to current methods for measuring respiration in macrobenthic invertebrates as they are generally applied: measurements are made in the laboratory, not in the field; they are usually carried out in clean sediment or in no sediment at all; and the animals typically are starved during the experiments. This study reports the first application of electron transport system (ETS) activity to estimate in situ rates of oxygen utilization in macrobenthic invertebrates. Measurements of ETS activity and oxygen uptake (R) were made on 2 intertidal species, the polychaete Nereis virens and the amphipodCorophium volutator, under natural conditions and during starvation. ETS activity was related to body size in both species in a manner typical for a metabolic function (ETS = a wP). R:ETS ratios were 0.09 for N. wrens and 0.42 for C. volutator (cf. a literature value of 0.49 for zooplankton) The lower ratio for the former may result from a more sedentary life style or the importance of anaerobic pathways to their overall energetics. The general trend of the starvation experiments for both species was that oxygen uptake declined over 10 to 12 d, but ETS activity remained relatively constant. C. volutator ETS activity was lower in early spring and consistently higher from early summer through fall while N. virens had its highest activity in the early spring and a lower but consistent level of activity in summer and fall; the changes may have been related to the abundance of food resources or to reproductive activity. ETS activity is potentially useful as an estimator of long-term variability in metabolic activity of marine macrofauna.