Rabbits were divided into 6 groups according to concentration of the test gases (O2-CO2-N2 gas mixture) used; 0%-5%-95%: 0% group, 1.0%-5.1%-93.9%: 1% group, 2.0%-10.3%-87.7% and 1.9%-20.6%-77.5%: 2% groups, 3.1%-5.1%-91.8%: 3% group, 4.1%-5.1%-90.8%: 4% group. In addition to the normocapnic groups, two hypercapnic groups (10% and 20% groups) were included. After an intravenous injection of urethane for anesthesia, the rabbit was fixed on its back on an operating table and the trachea was cannulated. Animals inhaled the test gases through a cylindrical unidirectional valve box connected to the cannula. Blood samples were drawn from the catheterized femoral artery. The length of time to the onset of apnea became shorter as the O2 concentration became lower. The 0%, 1%, and 20% CO2-added 2% groups became apneic within 1 min. The effect of severe hypercapnia was marked. The 20% CO2 group became apneic much sooner than the 10% CO2 group. The onset of apnea was much later in the 3% and 4% groups, and in these groups a steady state PO2-level was observed. The PO2 values in the steady state were 17 mmHg for the 3% group and 21 mmHg for the 4% group. The PO2 value at apnea was about 10 mmHg in the O%, 1%, and 10% CO2-added 2% groups. The effect of severe hypercapnia on this variable was also observed, the value in the 20% group being 15.8 mmHg. Apnea ensued immediately following a strong inspiratory effort which was diagnosed as apneusis.(ABSTRACT TRUNCATED AT 250 WORDS)