The redistribution of cardiac output on cold exposure in new-born rabbits.

1. In anaesthetized new-born rabbits isotope-labelled microspheres were injected into the left ventricle to investigate the distribution of cardiac output.2. On cold exposure the proportion of radioactivity (and hence of cardiac output) distributed to brown adipose tissue rose from 6 to 19%. Similar results were obtained in ten rabbits in which the distribution of microspheres labelled with two different isotopes was examined both in a neutral thermal and cold environment.3. The perirenal, inguinal, pectoral and axillary adipose tissue deposits showed a trivial increase in the proportion of cardiac output (from 1.2 to 1.9%) on cold exposure as compared with the cervical and interscapular deposits.4. The small intestine of new-born rabbits received 24% of the cardiac output while renal flow was only 8%. There were large changes in the proportions of body weight and in the distribution of cardiac output with age from 1 to 6 days from birth.5. The results support the contention that brown adipose tissue is the principal site of additional thermogenesis in anaesthetized new-born rabbits exposed to cold.