The occurrence of free amino acids in the amniotic and allantoic fluid of the cow and sow has recently been reported (Martinčić, Herak & Krvavica, 1969). In this study, the results obtained with the mare's allantoic fluid are recorded and show that substantial quantities of glutamic acid, glycine, alanine and proline and, in addition, a surprisingly large amount of hydroxyproline are present. This imino acid was also detected in the allantoic fluid of pregnant cows and sows, but in very much lower concentrations. Animal organs and body fluids contain, on the whole, very little hydroxyproline. In the blood, Wiss (1949) found an average of 0·132 mg/100 ml in rats, 0·171 in rabbits, and 0·290 in guinea-pigs; the quantities excreted daily in the urine were of the order of 10