THE phospholipase inhibitor quinacrine and the protein kinase C activator phorbol-12,13-dibutyrate were injected intracellularly into expiratory neurones of the ventral respiratory group within the brain stem of anaesthetized cats. Neurones were identified by their on-going spontaneous respiratory activity and by antidromic excitation from the spinal cord at the C2-C3 level. Phorbol-12,13-dibutyrate and quinacrine, increased the amplitude of respiratory drive potentials reproducing an effect which is also obtained by potassium blockers. We conclude that modulation accounts for approximately a 40% reduction of the excitatory respiratory drive potentials provided by the respiratory rhythm generator. This modulation appears to be mediated by potassium currents that are controlled by intracellular messengers in brain stem respiratory neurones.