We examined the respiratory rhythm of two in-vitro preparations from neonatal rats, the brainstem-spinal cord and transverse brainstem slice, recording the bursting activity of phrenic and hypoglossal nerves, respectively at 1 degree C intervals from 25 to 35 degrees C at two pH's, 7.4 and 7.1. In both preparations at either pH, burst frequency increased with temperature, burst duration declined and burst amplitude reached a peak at 30 degrees C. The shapes of the bursts changed from a decrementing pattern at low temperatures to a bell-shaped pattern at high temperatures. At reduced pH, frequency increased for temperatures between 25 and 32 degrees C in the brainstem-spinal cord but not in the slice. Burst duration was increased at reduced pH for temperatures between 27 and 29 degrees C in the brainstem-spinal cord, but not in the slice. Burst amplitude only changed with pH at the lower temperatures, decreasing at the lower pH in the brainstem-spinal cord and increasing in the slice. With respect to the effects of temperature, we concluded that both preparations were similarly affected, and that an increase in temperature alters the in-vitro burst pattern towards that observed in-vivo. With respect to the effects of pH, we concluded that effects differ between these in-vitro preparations and from in-vivo preparations, and that the difference between in-vivo and in-vitro preparations in their response to decreasing pH is not due to differences in temperature.