This paper reports the results obtained by the application of gas-liquid chromatographic techniques in determining the concentrations of 14 amino acids at different stages of the development of Walker 256 carcinoma which was implanted in 4 groups of Wistar rats. The increase in the levels of 6 of these amino acids was significant, and there was a correlation between the increase in the serum mino acid levels of rats with tumors, on the one hand, and, on the other, the ‘t:b ratio’ (where t = the weight of the tumor when transplanted and b = the total bodyweight of the recipient animal).