The marked seasonality of high latitude marine ecosystems means that the relationship between nutrition and metabolism is of particular interest, for many polar organisms must survive long periods without food. One hundred individuals of the scavenging lysianassoid amphipod Waldeckia obesa (Chevreux, 1905) were collected from Antarctica and then maintained in cold-water aquaria at the British Antarctic Survey in Cambridge. One group were fed and then starved for 64 days, during which time oxygen consumption fell to 60% of the initial value, and ammonia excretion to 20%. The initial decrease in metabolism occurred in about 5 days, after which time ammonia excretion remained roughly constant whereas oxygen consumption (and hence O:N atomic ratio) oscillated. O:N ratio and proximate composition estimated stoichiometrically from elemental composition both indicated the use of protein and lipid as metabolic substrates during starvation. Fed amphipods exhibited a classic post-prandial increase in metabolism (specific dynamic action, SDA): oxygen consumption and ammonia excretion increased rapidly to a level between 4 and 7.5 times the prefeeding levels, respectively. These are the largest increases of metabolic rate with feeding so far reported for a marine invertebrate and the overall SDA response lasted 8–10 days. O:N ratios indicated that metabolism 2–4 days after feeding was dominated by protein metabolism.