Four Karan/Friesian crossbred (Holstein×Tharparkar) dairy cows in late lactation (>200 days) were fed a basic diet of green maize and concentrates with an increasing proportion of Leucaena leucocephala (leucaena) leaf meal (LLM). The proportion was gradually increased from 0·25 of the dry matter intake (DMI) in the first week to 0·50 in the second, 0·75 in the third and ad libitum in the fourth week. Mimosine, 3,4-dihydroxy pyridine (3,4-DHP) and 2,3-dihydroxy pyridone (2,3-DHP) levels were determined in milk, serum, urine and faecal samples. On average DMI of leucaena was 0·023–0·025, 0·025–0·027, 0·027–0·028 and 0·022–0·025 of live weight (LW) during the first, second, third and fourth week, respectively. Mimosine, 3,4-DHP and 2,3-DHP appeared in the blood serum during leucaena feeding and continued appearing up to sixth week of experimental feeding even after the withdrawal of leucaena from the 34th day onwards. Similarly, excretion of mimosine, 3,4-DHP and 2,3-DHP were observed even after withdrawal of leucaena from the diet. The feeding of LLM resulted in a reduced level of T3 (Triiodothyronine) and T4 (Thyroxine) within a week of LLM feeding. The level of T3 and T4 improved to normal after withdrawal of LLM from the diet. The serum aspartate transferase (AST) and serum alanine transferase (ALT) activities were within the normal range. Leucaena feeding improved milk yield and composition only up to 3 weeks of feeding. The concentration of mimosine, 3,4-DHP and 2,3-DHP in milk was 0·33, 0·05 and 0·02 that of blood, respectively. The secretion of mimosine, 3,4-DHP and 2,3-DHP in the milk might be a concern for health of the offspring of leucaena-fed animals as well as human beings consuming such milk.