Horse heart was ground, washed with H2O, and extracted with an equal vol. of M/15 Na2HPO4 soln. for several hrs. Succino-, lacto-, malo- and glutamino-dehydrase were present in the extract. Glucose anaerodehydrase was prepared by precipitation with (NH4)2SO4 and dialysis of the acetone-H2O extract of horse liver. Glucose aerodehydrase was prepared by alco-hol-ether precipitation of a dialyzed and concd. Na2HPO4 extract of the mold Penicillium glaucum. Dimethyl-, butyl-and dibutylmalic acids and methyl-, ethyl-, butyl-, hexyl-, heptyl-, octyl-, nonyl-, decyl-, undecyl- and dodecylsuccinic acids were used as substrates or inhibiting substances. The anaerobic reaction mixtures contained 1-1.5 ml. enzyme soln., 0.5 ml. methylene blue soln. (1:5000), 0.5-1.5 ml. M/15 phosphate buffer, pH 7.3, and about 0.5-1 ml. of M/10 substrate or inhibiting substance. The aerobic mix-ture contained, in a total vol. of 4 ml., 0.5 g. wet tissue hash, 1-3.2 ml. M/15 phosphate buffer, and varying amts. of M/10 substrate or inhibiting soln. The anaerobic expts. were carried out in vacuo in the Thunberg apparatus, and the aerobic expts. under air in the Warburg apparatus. Alkyl malonic acids, in contrast to malonic acids, did not inhibit the action of succinodehydrase. Methyl- and ethylsuccinic acid were slowly dehydrogenated by succinodehydrase. In 2-4 fold excess, the alkyl succinic acids, from the octyl to the dodecyl derivatives, had inhibiting effects on succinodehydrase. The alkyl succinic acids had inhibiting effects on the acidodehy-drases and indophenol oxidase but little effect on glucose dehydrase.