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The effect of isotonic bicycle exercise on 12-adrenoceptors and cAMP production in lymphocytes was studied. The origin of the newly exposed receptors was studied by measuring radioligand binding in the presence of hydrophylic or lipophylic 'cold' ligands. During exercise ,32-adrenoceptor density increased from 1207 ± 144 to 1776 ± 152 sites/ cell (56 ± 14%). Isoprenaline induced an increase in cAMP synthesis, which was 68 ± 15% higher than those values measured before the test. Forskolin did not induce a higher increase in cAMP production after exercise. The increase in receptor density seems not to be due to a transfer of internalized receptors to the membrane surface. We conclude that physical exercise induces an up-regulation of ,32-adrenoceptors on lymphocytes and that these adrenoceptors are functional.