Changes of the blood lactate, pyruvate and plasma NEFA levels during the 40 g oral galactose tolerance test were investigated in 21 healthy subjects, 42 patients with liver diseases and 47 patients with diabetes mellitus.<BR>The blood galactose concentration was measured by the enzymatic method using the galactose dehydrogenase reagent prepared by Boehringer-Mannheim Co. The blood lactate and pyruvate were also determined by enzymatic methods. The plasma NEFA was estimated colorimetrically by a modification of Duncombe's method. The results of the galactose tolerance test were classfied into three groups based on the previously described criteria, namely 0-1 grade is normal, 2-4 grade moderately abnormal, and 5-6 grade far abnormal.<BR>In the normal group of the galactose tolerance test, the blood lactate levels increased markedly and reached the maximum point in 60 min. after the galactose loading and came down to the previous levels in 120 min. The blood pyruvate levels showed a transient increase as with lactate in both healthy and diabetes groups, but no significant changes in the liver diseases group.<BR>The relationship between the blood lactate and pyruvate levels during the galactose tolerance test showed a significant positive correlation in healthy subjects (γ=+0.465, p<0.01) and in diabetes subjects (γ=+0.568, p<0.01) but no significant correlation in the liver diseases subjects.<BR>In the moderately abnormal groups, the lactate and pyruvate levels after the galactose loading showed slight elevations in both liver diseases and diabetes subjects.<BR>In the far abnormal groups, increases of the blood lactate and pyruvate levels after the galactose loading were hardly observed.<BR>It was demonstrated that the results of the galactose tolerance test closely were related to the variation of the blood lactate and pyruvate levels after the galactose loading, therefore this test would be suitable for estimating the redox potential of the liver cytoplasma.<BR>Furthermore, in the abnormal groups, contrary to the normal group, a significant negative correlation between Δlactate and ΔNEFA during the galactose tolerance test were found in liver diseases subjects (γ=-0.343, p<0.01) and in diabetes subjects (γ=-0.363, p<0.01). Consequently it was suggested that a compensative relationship between the liver and the fatty tissue might be discussed on the galactose metabolism.