Changes in the composition and morphology of interscapular brown adipose tissue (BAT) are described for the adult pigmy mouse, Baiomys taylori, in its short-term, cold-torpor cycle. The study includes an analysis of tissues from, (1) room temperature (RT) controls, (2) RT, 24 hour starvation animals, ( 3 ) mice exposed on 1 5°C for six hours, (4) torpid animals (24, 48, and 72 hours at 15"C), and (5) mice allowed to arouse from torpor at RT for 1, 2, 5, 9, 24, and 288 hours. Control brown fat is composed predominately of lipid vacuoles, glycogen particles, and mitochondria with lamellaelike cristae. Starvation at RT has no marked effect on either the BAT energy stores, or the mitochondria1 morphometry. Short exposure to 15°C brings about a pronounced depletion in the lipid and glycogen content, and a significant swelling in the mitochondria. Brown fat from torpid mice is deplete of lipid and glycogen. Mitochondria remain enlarged, and have parallel, closely-packed cristae. The lipid and glycogen reappear in the early hours of arousal. The multilocular character of the control BAT is reached in five to nine hours after initiation of arousal. Throughout the arousal the mitochondria retain their cold-induced morphological a1 terations. Pinocytotic microvesicles in the capillary endothelium and in the brown adipocytes are observed in different degrees, i n all experimental groups. With the histological changes in evidence, the participation of pigmy mouse brown fat i n non-shivering thermogenesis in situations of cold exposure, and arousal from torpor, is discussed.