When aspartate- and alanine transaminase (AST and ALT, respectively) activities were studied in homogenates of rat cerebellum, brain cortex and brain stem (using a modified procedure by Raitman and Frenkel), addition of 50 microM pyridoxal-5-phosphate (PALP) increased 2-3-fold the activity studied. With an increase in PALP concentration from 6.25 microM up to 200 microM AST- and ALT-activities increased dose-dependently, while at 100-200 microM concentration of PALP saturation of the reaction occurred and 200 microM of PALP decreased the AST activity. 10 min preincubation of these homogenates without and in presence of 0.25 mM PALP led to a distinct increase in amount of ketoderivatives reacting positively with 2,4-dinitrophenyl hydrazine, most markedly elevated in the sample containing PALP. At the same time, prolongation of the preincubation period under standard conditions in presence of an excess of the substrates and 50 microM PALP did not cause any increase in AST- and ALT-activities. During studies of the transaminase activity in brain biopsy, in the material obtained after neurosurgical operations as well as in medico-biological experiments, the activity should be estimated using two sets of conditions: under standard conditions and in presence of 50-100 microM PALP but without preincubation with PALP.