|Abstract:||Three experiments were conducted to determine the effects of supplemental carnitine for lactating dairy cows. In the first experiment, carnitine (ca. 6 g/d) was administered into either the rumen or abomasum. Concentrations of carnitine in milk, plasma, liver, and urine increased similarly for both sites of carnitine administration. Milk yield and composition were not affected, except for solids-not-fat content and yield which increased when carnitine was administered at either site. Apparent total tract digestibility of fatty acids increased from 58.9% for controls to 61.3% when carnitine was administered at either site. In vitro measurements showed that less carnitine added to ruminal fluid obtained from a cow fed a high concentrate diet was degraded than in ruminal fluid obtained from a cow fed a conventional diet; degradation increased after 2 wk adaptation to 7.0 g/d of dietary carnitine in ruminal fluid obtained from cows maintained on either diet. In the second experiment, 0,.875, 1.75, 3.5, and 7.0 g/d of carnitine were included in the diet. Contents of total solids, fat, and protein in milk were affected cubically, such that feeding.875 or 7.0 g/d of carnitine increased component content of milk but feeding 1.75 and 3.5 g/d of carnitine decreased milk component content relative to the control treatment. Concentrations of carnitine in milk and plasma increased linearly and apparent total tract digestibilities of fatty acids decreased quadratically in response to increasing amounts of carnitine in the diet. In the third experiment, 0, 3, 6, and 12 g/d of carnitine were infused into the abomasum. Concentrations of carnitine in milk, plasma, and urine increased linearly in response to increasing abomasal carnitine; concentrations of carnitine in plasma and milk appeared to become saturated when 6 g/d of carnitine were infused into the abomasum. Milk yield and composition were unaffected and apparent total tract digestibilities of fatty acids tended to decrease linearly (62.6, 61.8, 59.8, and 59.7%, respectively) as more carnitine was infused into the abomasum. Based on the data from these experiments, there does not appear to be any benefit to providing carnitine in the diets of lactating dairy cows.