The objective was to find out whether the reduced amount of brown adipose tissue in myopathic hamsters [Am. J. Physiol. 239 (Cell Physiol. 8): C18-C22, 1980] was secondary to a refractoriness to the trophic influence of norepinephrine. However, no evidence for any trophic influence of norepinephrine on brown adipose tissue of either normal or myopathic hamsters could be detected under experimental conditions that have demonstrated such an influence in rats. A mediator other than norepinephrine, melatonin, secreted by the pineal gland, is known to mediate the control of brown adipose tissue growth induced in hamsters by short photoperiod. Further studies of intact or pinealectomized hamsters showed that the pineal gland was not required for either cold- or diet-induced growth of brown adipose tissue. It is concluded that the defect in control of brown adipose tissue size in the hamster with muscular dystrophy is not due either to abnormal control by norepinephrine or to the pineal gland since neither of these appears to participate in the normal regulation of brown adipose tissue size in relation to environmental temperature or to diet.