During mercurial intoxication produced by the intraperitoneal injection of 10 mg of mercury as mercaptomerine, total LDH activity decreases in the superficial and deeper cortex of the kidney. Increased LDH levels are seen in blood plasma while measurable amounts appear in the urine. The LDH isoenzyme pattern of the cortex – which contains predominantly the aerobic LDH1 and LDH2 fractions – shows a small relative increase of the anaerobic LDH5 fraction, visible after 24 h. Total LDH activity of inner medulla remains unchanged, its isoenzyme pattern, however, which normally contains predominantly LDH4 and LDH5 fractions, shows a shift towards an increase of the LDH1 and LDH2 fractions; these modifications are already present after 4 h. Changes in LDH isoenzyme pattern can be the expression of a disproportional loss or breakdown of the individual LDH fractions from the cells; they can also reflect a cellular adaptation to changed metabolic conditions. These possibilities are considered for the LDH1 and LDH2 increase seen in the LDH isoenzyme pattern in the inner medulla during mercury intoxication and for the LDH5 increase in superficial cortex 24 h after injection of mercury.