Telemetered heart rate (fH) was examined as an indicator of activity and oxygen consumption rate (VO2) in adult, cultivated, Atlantic salmon, Salmo salar L. Heart rate was measured during sustained swimming in a flume for six fish at 10° C [mean weight, 1114 g; mean fork length (f. l.), 50·6 cm] and seven fish at 15° C (mean weight, 1119 g; mean f. l., 50·7 cm) at speeds of up to 2·2 body lengths/s. Semi–logarithmic relationships between heart rate and swimming speed were obtained at both temperatures. Spontaneously swimming fish in still water exhibited characteristic heart rate increases associated with activity. Heart rate and Vo2 were monitored simultaneously in a 575–1 circular respirometer for six fish (three male, three female) at 4° C (mean weight, 1804 g; mean F. L., 62· cm) and six fish (three male, three female) at 10° C (mean weight, 2045 g; mean f. l., 63·2 cm) during spontaneous but unquantified activity. Linear regressions were obtained by transforming data for both fH and Vo2 to log values. At each temperature, slopes of the regressions between fH and Vo2 for individual fishes were not significantly different, but in some cases elevations were. All differences in elevation were between male and female fish. There were no significant differences in regression slope or elevation for fish of the same sex at the two temperatures and so regressions were calculated for the sexes, pooling data from 4 and 10° C. There was no significant difference in the mean ± S. D. Vo2 between the sexes at 4° C (male, 66·0 ± 59·6 mgO2 kg−1 h−1; female, 88·0 ± 60·1 mgO2 kg−1 h−1) or 10° C (male, 166·2 ± 115·4 mgO2 kg−1 h−1; female, 169·2 ± 111–1 mgO2 kg−1h−1). Resting Vo2 (x̄± s. d.) at 4°C was 36·7 ± 8.4 mgO2 kg−1 h−1, and 10° C was 72·8 ± 11·9 mgO2 kg−1 h−1. Maximum Vo2 (x̄± S. D.) at 4° C was 250·6 ± 40·2 mgO2 kg−1 h−1, and at 10° C was 423·6 ± 25·2 mgO2 kg−1 h−1. Heart rate appears to be a useful indicator of metabolic rate over the temperature range examined, for the cultivated fish studied, but it is possible that the relationship for wild fish may differ.