1. The effects of a number of respiratory inhibiting agents on the cell division of fertilized eggs of Arbacia punctulata have been determined. For eggs initially exposed to the reagents at 30 minutes after fertilization at 20 degrees C., the levels of oxygen consumption prevailing in the minimum concentrations of reagents which produced complete cleavage block were (as percentages of the control): In 0.4 per cent O(2)-99.6 per cent N(2), 32; in 0.7 per cent O(2)-99.3 per cent CO, 32; in 1.6 x 10(-4)M potassium cyanide, 34; in 1 x 10(-3)M phenylurethane, 70; in 4 x 10(-3)M 5-isoamyl-5-ethyl barbituric acid, 20; in 3 x 10(-4)M iodoacetic acid, 53. 2. The carbon monoxide inhibition of oxygen consumption and cell division was reversed by light. The percentage inhibition of oxygen consumption by carbon monoxide in the dark is described by the usual mass action equation with K, the inhibition constant, equal to approximately 60, as compared to values of 5 to 10 for yeast and muscle. In 20 per cent O(2)-80 per cent CO in the dark there was a slight stimulation of oxygen consumption, averaging 20 per cent. 3. Spectroscopic examination of fertilized and unfertilized Arbacia eggs reduced by hydrosulfite revealed no cytochrome bands. The thickness and density of the egg suspension was such as to indicate that, if cytochrome is present at all, the amount in Arbacia eggs is extremely small as compared to that in other tissues having a comparable rate of oxygen consumption. 4. Three reagents poisoning copper catalyses, potassium dithio-oxalate (10(-2)M), diphenylthiocarbazone (10(-4)M), and isonitrosoacetophenone (2 x 10(-3)M) produced no inhibition of division of fertilized Arbacia eggs. 5. These results indicate that the respiratory processes required to support division in the Arbacia egg may perhaps differ in certain essential steps from the principal respiratory processes in yeast and muscle.