Male albino mice, with and without transplanted neuroblastoma C1300, had tissues analyzed for prostaglandins by radioimmunoassay. Highest prostaglandin concentrations occurred in tumor tissue, lungs, spleen, and kidneys. Tritiated prostaglandins A1 or E1 and tritiated arachidonic acid were injected intravenously. The concentration (organ/tumor) was higher at 1 hr than at 6 or 24 hr. Prostaglandin E1 and arachidonic acid localized principally in the kidney (likely the excretory organ) whereas prostaglandin A1 showed highest concentration in the liver.