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Divergent selection for heat production/ loss (kcal•kg-.75•d-1) , measured in 9- to 11-wk-old male mice, was conducted for 15 generations. Heat loss was measured for 15 h on individual animals placed overnight in direct, gradient-layer calorimeters. Selection for high (MH) and low (ML) heat loss and unselected control (MC) occurred in each of three replicates for a total of nine unique lines. Repeatability of the heat loss measurement was .45 and the CV was 10.5%. Cumulative realized selection differentials, averaged for the three replicates, were 145.1 and -105.0 (kcal•kg-.75•d-1) and ranged from 136.9 to 149.2 and -107.1 to -101.3 for MH and ML selection, respectively. Cumulative standardized realized selection differentials, averaged for the three replicates, were 10.06 and -9.51 for MH and ML selection, respectively. Direct responses (kcal•kg-.75•d-1) in heat loss after 15 generations were 44.2 for MH and -27.4 for ML as deviations from MC. Asymmetry of response was evident (P = .03) by Generation 10. Realized heritability was .28 ± .01 based on divergence of MH and ML selection. For selection for higher and lower heat loss, realized heritabilities were .31 ± .01 and .26 ± .01, respectively.