ABSTRACT O2 uptake was determined for periods of 23–46 h in salt-depleted crayfish held in deionized water (DW) or Na-free media at 10°C. These media were replaced by artificial lakewater media (ALW) containing 0·20–6 mM Na and O2 uptake was again determined for periods of 24–66 h. During net ion uptake in ALW the metabolic rate was either elevated or depressed. Standard metabolism in ALW altered by amounts equivalent to 0·1–15·5% (mean 6·4 (15)14·4% s.D.) of the metabolic rate measured during salt-depletion. On three occasions the metabolic rate was elevated by 22·0–66·7%, but some of this increase may have been due to locomotor activity. The calculated values for thermodynamic work involved in ion transport were 0·056–0·268 J/10 g.h at 10°C, or 1·5–7·2% of the mean standard metabolic rate. Most of the observed changes in metabolic rate lie within the limits of experimental error (ca. ± 7 %). Hence the energetic cost of ion transport is too small for direct measurement in intact crayfish.