Exposure to short photoperiod increased the activity of adrenal tyrosine hydroxylase (TOH). During summer, the shortening of photoperiod at constant Ta of 23 degrees C stimulated TOH from 7.28 to 10.94 nmoles L-Dopa (h . pair of adrenals), which was as effective as a chronic exposure to 15 degrees C Ta, elevating TOH to 9.1 nmoles L-Dopa/(h . pair of adrenals). The stimulation of TOH by photoperiod or cold exposure in Djungarian hamsters was well correlated with an increase in their ability for nonshivering thermogenesis.