The post-natal changes in minimal metabolic rate in the pig.

1. Minimal metabolic rate was measured as oxygen consumption in new-born pigs when environmental temperature was rising and passing through thermal neutrality.2. Three different periods were observed in 20 days after birth. The first period included the time from birth to 82 hr and was characterized by a continuous rise in minimal metabolic rate from 9.4 to 16.0 ml. O(2)/kg.min (70% increase). The second period extended from 82 hr to 9.5 days with values near to 16 ml. O(2)/kg.min. The third period beginning on the eleventh day lasted until observations ceased on the twentieth day and was characterized by a 25-30% fall in minimal metabolic rate when compared with the second period.3. Minimal metabolic rates from the second and third period were proportional to body weight and not to the surface area. Exponential factors for body weight were 0.93 and 0.96 for the second and third periods respectively.4. Rectal temperature rose from 38.6 degrees C on the first day to 39.3 degrees C on the second and 39.5 degrees C on the fourth day after birth.5. Environmental temperature associated with minimal metabolic rate showed a general tendency to fall from 36.5 in the first 4 days to between 33 and 34 degrees C in the following days.6. Age rather than body weight is the decisive factor influencing changes in minimal metabolic rate in the new-born pig.