Experiments were performed on 44 urethane anesthetized, paralyzed, bilaterally vagotomized and artificially ventilated rabbits. Unilateral microinjection of different concentrations of tutin (0.1 microgram/microliter, 1 microgram/microliter, 5 micrograms/microliters, 10 micrograms/microliters, n = 8 for each group) into the nucleus tractus solitarius (NTS) region could induce reversible increase in respiratory frequency in the phrenic neurogram due to the shortening of inspiratory and expiratory duration and a decrease of the peak amplitude of the integrated phrenic nerve activity. However, no significant effects on arterial blood pressure (BP), heart rate (HR) and electroencephalogram (EEG) were noted. Of the four concentration groups of tutin application, 1 microgram/microliter group already attained a maximal effect. Microinjection of 5 micrograms/microliters and 10 micrograms/microliters tutin into NTS region produced further a disturbance of respiratory rhythm exhibited as transient phrenic nerve discharge during expiratory phase or irregular prolongation of expiratory duration. Unilateral injection of 1 microliter normal saline into NTS region (n = 8) had no significant effects on phrenic activity, BP, HR and EEG. The results suggest that NTS area might be involved in the transition of respiratory phases in the rabbit, and that tutin has a direct stimulating action on the respiratory phase switching mechanism of the NTS region, thus facilitating the transition of respiratory phases with an increase of respiratory frequency.