The ventilatory response to CO2 in rats under sodium pentobarbital anesthesia has been measured using the rebreathing technique. The animal rebreathed through a tracheal cannula for a period of 4 min from an apparatus of 200-400 ml capacity, containing 5-6% CO2 in O2. Pco2 in the rebreathing apparatus (PappCO2), instantaneously Vt, f, and Ve were monitored before, during, and after rebreathing. During the rebreathing run, PappCO2 and Paco2 rose linearly from 35-40 to 65-70 mm Hg; there was no significant difference between PappCO2 and Paco2 at any time during rebreathing. Vt and Ve increases almost linearly with the rise of PappCO2, while f increased to a maximum within 2 min of rebreathing. In the rat, Vt regulation seemed to operate exclusively as a proportional control system in response to linearly increasing CO2 stimulus. The slopes of PappCO2, Vt or Ve response curves varied considerably during the time course of the experiment, depending upon the level of anesthesia, even though there was no large change in Ve in the control periods which were under hyperoxic conditions. However, a significant linear relationship was seen between f in the respective control period and the slope of PappCO2-Vt response at various levels of anesthesia. We concluded that the rebreathing technique can be applied in small experimental animals and that changes in the sensitivity of the respiratory control system to a CO2 stimulus by anesthesia can be easily monitored by repeating the rebreathing test.