We investigated the extent of catecholamine (CA) conjugation in plasma and accumulation inside red blood cells (RBCs) after forced immobilization of the rat. A control blood sample was obtained from undisturbed rats resting in home cages via an indwelling aortic catheter. Then rats were immobilized for 2 h, and blood samples were taken during stress at 15 min and 2 h and at 30 min poststress. Both sulfate and glucuronide conjugates were deconjugated by enzymatic hydrolysis and measured by radioenzymatic thin-layer chromatographic methods. In plasma of resting rats, the major conjugate of norepinephrine (NE) is sulfate and that of epinephrine (E) and dopamine (DA) is glucuronide; total conjugates in plasma are 53, 57, and 97% of total NE, E, and DA, respectively. During stress, sulfate conjugates of NE, E, and DA increased significantly along with free NE, E, and DA. DA glucuronide unexpectedly declined at 2 h of stress and 30 min poststress. Concentrations of free CAs rose in RBCs during stress, as did NE sulfate, the only conjugate present in RBC lysate. We conclude that both CA conjugation and RBC accumulation are enhanced when large amounts of free CA enter the circulation as a result of immobilization stress and that the relative importance of these inactivation pathways is different for the three CAs.