Insulin (100-3333 microU/ml) stimulates iodothyronine 5'-deiodinase 3 to 4 fold in dispersed rat brown adipocytes. Deiodinase activity increased steadily from 1 to 4 hours. Insulin increased enzyme activity via an increase in the Vmax while the Km remained unchanged. Omission of glucose from the medium did not affect the insulin response. Studies with alpha-amanitin suggested that the increase in deiodinase activity was not due to an increase in the rate of transcription. The insulin effect was not additive to that of alpha 1-catecholamines, suggesting the two stimulators might have one or more common elements.