Brown adipose tissue (BAT) non-shivering thermogenesis is stimulated by cold temperature and depressed by hypoxia. We investigated the extent to which changes in metabolic rate during cold and hypoxia, singly or combined, were accompanied by changes in BAT perfusion. One-month-old rabbits were instrumented for measurements of regional blood flow by the coloured microsphere technique. One group of rabbits was tested in warm (24 degrees C, n=17), and the other in cold (13 degrees C, n=9) conditions, first in normoxia (inspired oxygen concentration FIO2 about 21%, arterial oxygen saturation SaO2 approximately 88%) followed by hypoxia (FIO2 approximately 10%, SaO2 approximately 54%). In warm conditions, oxygen consumption (VO2, measured by an open-flow method) averaged 22 ml.kg-1.min-1 (STPD), and BAT blood flow 98 ml.100g-1.min-1. In hypoxia, VO2 dropped on average to 87%, whereas BAT flow dropped to 43% of the normoxic values. In the cold during normoxia, VO2 averaged 31 ml.kg-1.min-1 (STPD), and BAT blood flow was 155 ml.100g-1.min-1. In cold and hypoxia VO2 dropped to 19 ml.kg-1.min-1 (STPD) (i.e. 60% of the normoxic value), whereas BAT blood flow was not altered significantly (148 ml.100g-1.min-1). Hence, BAT blood flow decreased in hypoxia in absence of cold stimuli, whereas it remained high when hypoxia occurred during cold, despite the major drop in VO2. We conclude that cold is more important than hypoxia in determining BAT perfusion, and that changes in BAT blood flow are not a mechanism for the hypoxic control of V.O2.