The present study was designed to investigate the effects of oral L-carnitine administration on different blood parameters (hematocrit, lactate, glucose, cholesterol and carnitine content, activity of creatinkinase, aspartat aminotransferase, lactate dehydrogenase) and on the heart rate of eventing horses. For this purpose two groups of eight horses each were tested by five exercise trials conducted during one sport season from March to August. The experimental group was given 5 g L-carnitine daily with oats. All horses were trained in the same way using a standardized programme. Every four to five weeks the physical state was tested during a standard exercise (2.2 km trot uphill, incline 200 m, time 7 min); this run had to be done twice. During the second run the heart rate as well as the time was measured. Blood samples were taken before as well as immediately after the physical examination. The results of lactate level in serum, heart rate and time for the standardized track did not show any influence of carnitine on the performance of the horses. Plasma contents of free carnitine as well as of carnitine esters in the trial group were significantly higher than in the control group. In the trial group the total carnitine content increased slightly during the examination. Before the test run the values of the different performance tests in the trial group were between 27.4 (n = 8; SD = 6.1) and 40.5 mu mol/l (n = 7; SD = 11.0), and after the exercise between 29.1 (n = 8; SD = 9.1) and 52.2 mu mol/l (n = 7; SD = 14.7). In the control group the values before exercise varied between 16.0 (n = 6; SD = 3.6) und 28.0 mu mol/l (n = 6; SD = 4.5), immediately after the test run between 19.3 (n = 6; SD = 3.4) and 28.1 mu mol/l (n = 6; SD = 5.0). For the second trial four eventing horses, at the beginning of their interval training, were tested in irregular intervals (the weather did not permit the test runs every week) on a standardized track (3.2 km, 270 m incline, time 9 min). The first two test runs were developed without carnitine supplementation, afterwards two horses were given 5 g L-carnitine/day for two weeks; performance tests were done twice during this time. After these two weeks the two other horses got L-carnitine daily and the performance tests were done twice again. There was no influence of L-carnitine on the parameters serum lactate and heart rate.