Effects of intracisternal administration of the gamma-aminobutyric acid receptor agonist, muscimol, on recurrent laryngeal and phrenic nerve activities were assessed in the cat. Muscimol (0.1-2.4 micrograms) produced a dose-related inhibition of the respiratory activity in both of these nerves. The inhibition was seen as a decrease in the amplitude of nerve activity with no significant change in respiratory rate. The ED50 values for muscimol and its 95% confidence interval for inhibition of the inspiratory activity in the recurrent laryngeal and phrenic nerves were 0.22 micrograms (0.17-0.29) and 0.27 micrograms (0.19-0.38), respectively. The potency of muscimol for inhibition of these nerve activities was not significantly different. Expiratory activity in the recurrent laryngeal nerve was completely inhibited prior to either the inspiratory activity in this nerve or in the phrenic nerve. Intracisternal administration of the gamma-aminobutyric acid receptor antagonist, bicuculline, rapidly reversed the inhibition in both nerves. Reversal times for inhibition of the inspiratory activities in the recurrent laryngeal and phrenic nerves were not significantly different. Return of inspiratory activity in each nerve always preceded the return of expiratory activity in the recurrent laryngeal nerve. These results demonstrate that CNS gamma-aminobutyric acid receptors are involved in the regulation of respiratory motor outflow to the larynx as well as the diaphragm.