AbstractThe effect of immobilization, cold and immobilization plus cold stress on rat pineal indoles and brain serotonin (5‐HT) has been studied. Only combined immobilization and cold stress resulted in an increase of pineal melatonin and N‐acetylserotonin (NAS). Furthermore, the ratio of 5‐hydroxyindole acetic acid (5‐HIAA) to 5‐HT was significantly decreased in rats subjected to the combined stress. This effect was evident in the pineal as well as the hypothalamus and brainstem. Although showing no effect on melatonin, NAS, or the 5‐HIAA/5‐HT ratio, immobilization alone produced a significant increase in pineal 5‐HT concentration. This 5‐HT increase was also evident in the hypothalamus and brainstem. Synaptosomal 5‐HT uptake was examined as a functional test of 5‐HT activity, but was unchanged by the combined stress. Although cold alone did not significantly affect serum corticosterone levels, it did augment the corticosterone increase induced by immobilization. These results are discussed in the context of previous investigations of the endogenous monoamine oxidas