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The effect of sympathetic activity on T4 and T3 levels in cold-exposed rats was investigated. Administration of the highest dose of propranolol (2 mg/100 g b.w.) twice daily during 4 days decreased T4 and T3 concentrations in plasma of rats living at 23°C (T4 from 46.4 ± 2.6 to 25.8 ± 5.3 nmol/l and T3 from 1.08. ± 0.6 to 0.82 ± 0.12 nmol/l). No significant effect on T4 and T3 levels (49.0 ± 11.6 and 1.48 ± 0.16 n/mol, respectively) after the administration of the same dose regimen of propranolol was observed in rats exposed to cold for 4 weeks. T4 and T3 levels in rats exposed to cold for 4 weeks were not significantly altered 1 week after sympathectomy, while remaining in the cold. However, chemical sympathectomy before cold exposure delayed the cold induced T3 elevation occurring during the first week of cold exposure (controls: from 1.16 ± 0.19 to 1.44 ± 0.29 nmol/l; sympathectomized rats: from 1.07 ± 0.12 to 1.17 ± 0.22 nmol/l). After 2 weeks of cold exposure the T3 levels of controls and sympathectomized rats were not significantly different (controls: 1.45 ± 0.12 nmol/l, sympathectomized rats: 1.38 ± 0.15 nmol/l). No effect of sympathectomy was observed on T4 levels. These experiments show that the role of sympathetic activity in increasing T3 is not clear during cold exposure. They provide some evidence that sympathetic activity may play a role in the initiation of the process leading to increased T3 plasma levels during cold exposure.