The level of pyridine nucleotides (NADH and NAD+) in the mineralizing growth plate of the chick was ascertained by high-resolution scanning microfluorimetry and biochemical analysis. Scanning electron microscopy and light microscopy were used to relate the concentrations of NADH and NAD+ to stages of chondrocyte maturation. A dramatic increase was found in the relative concentration of reduced pyridine nucleotides in the hypertrophic zone. On either side of this zone, in proliferating and calcifying cartilage, there was a decrease in NADH fluorescence, and the NADH/NAD+ ratio was depressed. The finding that NADH accumulated in the tissue zone associated with the earliest deposition of bone mineral supports the hypothesis that a change in the redox state initiates tissue mineralization.