SummaryTissue distribution and excretion of radioactivity following intravenous injection of Mn54Cl2 at 3 different time intervals were studied in rats deficient in pyridoxine and in control animals. The findings indicate that deficiency of pyridoxine is associated with increased uptake of the injected dose by brain and testicles, and with decreased uptake by liver and heart. No differences in Mn54 concentrations were observed in other tissues examined. The tracer content in the gastrointestinal tract and the feces of the pyridoxine deficient rats is significantly higher than in those of their respective controls. However, urinary excretion of radioactive manganese is reduced in pyridoxine deficient groups. These results suggest a role of pyridoxine in the regulation of manganese metabolism.