The concentration of 11-OCS in the blood plasma increased 2 weeks after thymectomy of adult rats; 3 to 4 weeks later this index fell to subnormal values. The following changes were observed in the spleen 2 to 4 weeks after the operation: a reduction of P-32 incorporation into the RNA and particularly into the DNA and also a fall of the ATP phosphate and lactic acid concentration with a simultaneous increase of P-32 incorporation into the whole tissue and of the K/Na ratio. With some reduction of ATP phosphate and in the absence of any increase in P-32 incorporation into the whole tissue there was an increase in P-32 incorporation into the RNA. The absence of such metabolic changes in case of false thymectomy and following splenectomy pointed to their specific dependence on the exclusion of the thymus function. Dynamics of these shifts failed to correlate with the changes in the 11-OCS content in the majority of the tissues.