Metabolic changes and toxic effects in Crangon crangon (collected during summer 1989 near Helsingor, Denmark and at Kulhuse in the Isefjord) were observed during exposure to different arsenate concentrations. Survival, respiration, and concentrations of hemolymph glucose, muscle glycogen and adenine nucleotides were measured. C. crangon was very tolerant of arsenate; survival was only affected at arsenate concentrations > 25 ppm. Small individuals (0.2 to 0.29 g wet weight) were less tolerant than medium-sized and large individuals (0.5 to 0.59 g and 0.8 to 0.89 g wet weight), e.g. at 50 ppm the LTs0 for small individuals was 180 h compared to 343 h and 360 h in middle-sized and large individuals. The relation between size and toxicity is probably due to the change in surface/volume ratio. Under clean conditions the respiration of C. crangon showed a clear relation between routine IVIO2 (0 2 in mlg -1 h -1) and wet weight (r2= 0.89; P < 0.001). During shortterm exposure to arsenate (10 and 50 ppm), the respiratory rates (1~IO2) seemed to decrease in a size-dependent manner. The concentrations of blood glucose and muscle glycogen did not change in response to arsenate exposure. The concentrations of adenylate phosphates were high and constant throughout the experiments. C. crangon had maximal adenylate energy charge (AEC) values of ca. 0.8. No decrease in AEC due to the exposure to arsenate was observed.