The discharge of cat's bulbar respiratory neurones (RN) was shown to be modulated by periodic depressions which are characterized by their ability to reduce the effectiveness of microiontophoretically applied depolarizing agents: L-glutamate, acetylcholine and potassium. From the observation of cycle triggered time histograms (CTH), it appeared that these depressions have a determined and invariable phase relationship within the respiratory cycle. They were demonstrated in RN histologically located between and including the nucleus of the tractus solitarius and the nucleus ambiguus. Reproducibility and dose/response relationship of L-glutamate-induced depolarizations enabled an estimation of the functional effectiveness of these periodic depressions. In spontaneously phasic or 'silent' RN, depressions were demonstrated in the majority of cases (71%). Strongest depressions prevented spontaneous and L-glutamate-induced firing. Slighter depressions did not completely abolish L-glutamate effectiveness but reduced it by 20-90%. Conversely, in the majority of spontaneously tonic units (68%) depressions were not identified since the L-glutamate effect remained unchanged throughout the respiratory cycle. Four types of these respiration-related depressions were differentiated on the basis of their length, their phase relation to the respiratory cycle and their potentiation in barbiturate-anaesthetized preparations. A first type suppressed L-glutamate-evoked firing throughout inspiration; it was found in late-expiratory neurones. Two other types of depressions had a more restricted duration in the cycle: one was restricted to a portion of inspiration and was found in early-expiratory neurones; the other restricted to the beginning of expiration, was found in a special group of inspiratory neurones. A fourth type of inhibition was weaker and actively prolonged throughout expiration: it was found in another group of inspiratory neurones including the respiratory neurones located at the level of the nucleus of the tractus solitarius. These periodic depressions are interpreted in terms of synaptic inhibition; it is proposed that they play a major role in the functional organization of the respiratory centers at the bulbar level.