The concentration of somatomedin A (SMA) in various rat tissues was determined directly by the radioreceptor assay in supernates of tissue homogenates at 100, 000×g for 60 min. The SMA concentrations in pancreas, kidney, lung, liver, spleen, testis, brain and muscle were 2.56, 2.40, 2.28, 2.16, 1.56, 1.44, 0.60 and 0.30U/g of wet tissue, respectively. The SMA content in rat serum was 6.66U/ml. The SMA concentration in tissues never exceeded that in serum. When the100, 000×g pellet of liver was treated with a hypotonic solution, the SMA content in the hypotonic solution was about 35% of that in the initial supernate. This result indicates that about30% of the SMA in liver was stored in organellae. SMA in tissues decreased with incubation at 37°C, though SMA in serum did not. This decrease could be precluded by adding 0.75mm HgCl₂ to the liver homogenate. The SMA content of the liver homogenate was dependent on growth hormone in experiments both in vivo and in vitro.