Liver and muscle glycogen, and blood 3-hydroxybutyrate concentrations were studied during and for 2 h after treadmill running for 1 h, in 144 carbohydrate-starved trained and untrained rats. The resting liver glycogen concentration of the trained animals was 227 ± 8 (mean ± S.E.M.) µmol glucosyl units/g wet mass, compared with 162 ± 12 µmol/g in the untrained animals. The muscle glycogen levels were 42 ± 1 and 28 ± 1 µmol/g respectively. Exercise reduced muscle and liver glycogen concentrations by approximately the same absolute amounts in both animal groups, leaving the trained rats with nearly 3 times as much residual glycogen as the untrained animals. There was very little resynthesis of muscle glycogen recovery, but the trained animals replenished approximately 43% of the liver glycogen used during exercise. The blood 3-hydroxybutyrate concentrations were negatively correlated with the simultaneous liver glycogen concentration of our experimental animals (r = −0·55; P 〈 0·001). It is concluded that trained animals primarily owe their resistance to post-exercise ketosis to their large stores of glycogen.