An assay procedure for a succinate-neotetrazolium-reductase system.

pregnancy to late lactation, whereas cytochrome oxidase remained constant. No change in the distribution of these enzymes or of protein was found at the three stages of the lactation cycle studied. 3. In mammary gland succinate-neotetrazolium reductase increased rapidly over parturition and throughout lactation; cytochrome oxidase showed a smaller increase during lactation. These increases were accompanied by changes in the enzyme. distribution among the mitochondrial and soluble fractions, more activity being found in the larger particles as lactation proceeds. 4. Mammary-gland mitochondria appear to change in size over the lactation cycle, being smallest in late pregnancy and largest in late lactation. In late lactation they sediment at approximately the same g-min. values as do liver mitochondria.