The present study was designed to investigate the effects of oral L-carnitine administration on different blood parameters (lactate, glucose, aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, creatine kinase) and on the heart rate of untrained beagles under physical exercise stress. Carnitine was administered (1 g per os; 71 to 106 mg/kg BM) on the days of scheduled tests, 3 h prior to the beginning of each test. Dietary carnitine content was about 20 mg daily. Blood samples were taken 1 h before the 20-min treadmill exercise as well as immediately after running and 20 min after the test. At 10, 15 and 19 min of running and again 5, 10 and 15 min after the exercise, the heart rate was recorded during 1 min using an electrocardiograph. Measured heart rates varied between 165 and 195 beats/minute during exercise with an average decrease within the first 5 min of rest of 33 to 48 beats/minute. Lactate levels did not increase or increased only slightly during exercise. The plasma carnitine contents assessed in three dogs who had not received any L-carnitine supplementation prior to the treadmill test averaged 17.74 mu mol/l of free L-carnitine and 19.32 mu mol/l of total carnitine before the rest, 14.64 and 21.57 mu mol/l immediately after the exercise and 16.75 and 23.76 mu mol/l 20 min after the test. When supplementing with L-carnitine, plasma levels showed increased values of 76.8 (free L-carnitine) and 86.0 mu mol/l (total carnitine) before the test, 61.8 and 151.7 mu mol/l, respectively, immediately after the exercise and 98.45 mu mol/l of free L-carnitine and 159.5 mu mol/l of total carnitine (free and esterified) 20 min after completion of the 20-min treadmill exercise session. No influence of L-carnitine supplementation on heart rate and blood parameters was observed.