Morphological and cytokaryometric examinations of the somatotropic cell population of the rat pituitary anterior lobe were performed on hypokinetic days 30, 90 and 165, and 60 days after 165-day hypokinesia (readaptation period). During prolonged hypokinesia the major changes occurred in the cytoplasm of somatotropic cells and were accompanied by an inhibition of the growth hormone synthesis. A higher activity of somatotropic cells during readaptation suggested that diminished motor activity may be responsible for a lower hormone production in the pituitary. The experimental findings and published data allow the conclusion that the inhibition of animal growth which takes place during hypokinesia is closely associated with an inhibited somatotropic function.