The uptake of radioactive copper by the brain and other tissues of the developing rat.

I. 64Cu as copper chloride in aqueous solution was given by intraperitoneal injection to rats varying in age from a few hours to 14 weeks. The isotope was allowed to circulate in the body for 24 h. 2. The amount of gamma radioactivity present in the brain and blood was measured and the percentage of the initial dose present was calculated. It was shown that the brain 64Cu activity reached a maximum around the 16th day of life and that the blood showed a steady decrease in the 64Cu activity per g from birth to maturity. Measurements were also made on the liver and kidney. 3. The excretion of g4Cu in the urine and faeces in 24 h was also studied. Approximately 30 yo of the isotope was excreted in that time, mostly in the faeces. 4. A limited number of experiments in three different age groups were carried out to discover whether changes in specific activity of the isotope given to rats had a significant effect on the percentage of 64Cu taken up by the brain and blood. No such effect was seen.