To examine the breathing rhythm-generating mechanism, effects of brain sectioning, immobilization, and electric stimulation on medullary respiratory activities were investigated in adult lampreys. The rostral part of the medulla (rostrally to the level of the caudal border of "internal acoustic pore") is not indispensable for breathing rhythm-generation. The rostral part itself, however, was also capable of driving periodic movement of only the first branchial baskets. After immobilization, respiratory discharges continued without changing their pattern, indicating that respiratory afferents do not modulate the centrally generating rhythm. Respiratory discharges recorded simultaneously from the right and left side of the medulla showed bilateral synchronization. After sectioning the midline of the brain, each of the symmetric halves of the medulla behaved as an independent respiratory pacemaker. Respiratory discharges were driven in one-to-one fashion by electric stimulus applied to the medulla, almost independently of timing of stimulus delivery. Stimulus pulses applied during respiratory discharges did not inhibit these discharges: electrically driven discharge summated or fused with the spontaneous firing. Slow and smoothly depolarizing potential preceding respiratory spike discharges was recorded intracellularly from the half of the brain-stem divided by midline sagittal sectioning in the immobilized animal. These results were discussed in light of the hypothesis that respiratory burst generator mechanism in the lamprey may be similar to cardiac pacemakers.