A comparison was made in 5 horses of the effects of administering 10 g L-carnitine by i.v. injection, nasogastric feeding or addition to the feed. The systemic bioavailability (reflecting intestinal absorption and hepatic first pass elimination) of L-carnitine administered orally was 7.1 (s.d. 5.3%). Oral administration doubled the plasma carnitine concentration after 4 h with a return to near normal levels by 12 h. Oral feeding did not increase 24 h urinary carnitine output suggesting that the plasma concentration did not exceed the renal threshold, with consequent retention in the tissues. Disappearance of plasma L-carnitine administered i.v. followed a 2-compartment model. The half time of disappearance was initially (t>0.5 α) 18–23 min decreasing to 115–176 min over the second phase (t>0.5 β). Administration i.v. (but not per os) resulted in a significant increase in plasma acetylcarnitine (and possibly also acylcarnitine). Eighty to 90 per cent of the administered dose was eliminated through the urine as L-carnitine within 24 h. Continued i.v. injection of 10 g/day for 26 days did not significantly affect the total L-carnitine content of the middle gluteal muscle in horses maintained under light exercise. The results are consistent with the normal plasma carnitine concentration in the mature well fed horse being sufficient to saturate transport into skeletal muscle and, as a result, this tissue compartment is relatively refractory to change through supplementation.