The aim of the present study is to investigate the influence of various experimental conditions, i.e., two different concentrations of D-fructose as a rapidly glycating substance and three incubation temperatures, on the glycation reaction of alanine aminotransferase (ALT, EC 2.6.1.2) and aspartate aminotransferase (AST, EC 2.6.1.1) expressed by decreasing catalytic activities of the enzymes during a 56 day in vitro incubation period. D-fructose in the concentration of 50 mmol/l did not inhibit the catalytic activity of either enzyme at 4 degrees C, partially inhibited AST activity in samples incubated at 25 degrees C (to 40% of the initial activity), and completely inhibited this enzyme at 37 degrees C at the end of the incubation period. In the presence of the same concentration of D-fructose, ALT showed no catalytic activity after 35 days at 25 degrees C or after 10 days at 37 degrees C. In 500 mmol/l D-fructose, complete AST inhibition was observed after 35 days (25 degrees C) or 20 days (37 degrees C), and no ALT activity was found on day 20 at either 25 degrees C or 37 degrees C. Taking into account the highest possible stability of enzymes, we suppose that a three-week observation of their residual catalytic activity in the presence of 50 mmol/l D-fructose at the temperature of 25 degrees C seems to be the most prospective experimental design for future glycation studies with aminotransferases under the influences of drugs.