Uterine fluid and blood serum samples were collected from 100 adult dairy cows during various stages of the oestrous cycle and analysed for their free amino acid content. A total of twenty-five free amino acids and amino compounds were identified in uterine fluids while only twenty-three were identified in blood serum. The concentration of the total and of most of the individual free amino acids was greater in uterine fluid than in blood serum at all stages of the oestrous cycle. The total content of free amino acids and the concentration of several individual free amino acids in uterine fluids showed cyclical variations. The significance of free amino acids in uterine fluids is discussed.