The neural control mechanism of pharyngeal constrictor (PC) activity of rabbits was examined in relation to vagal afferents. The average firing rate of single PC units of unilaterally vagotomized animals increased from 0.52 to 8.83 spikes/sec by cooling of the intact vagus nerve at 5 degrees C. Time courses of increases in PC and diaphragmatic (Dia) activities were examined during the period from the start of vagal cooling to the time when the PC reached the maximum firing rate. Although Dia activity linearly increased along with the respiratory cycles, PC activity slowly increased during the first three-quarters of the period and rapidly increased during the last quarter. The result suggests that more impulses in the vagus nerve are required to be blocked by cooling at a lower temperature for eliciting an increase in PC activity compared with that in Dia activity. The increase in PC activity evoked by bilateral vagotomy (from 0.30 to 10.07 spikes/sec) was not different from that by vagal cooling. The presumable afferent nerve fibers responsible for the increase in PC activity were discussed.