Mucociliary transport in the rat trachea was monitored through a surgical window for approximately one hour. Rates were grouped according to 5-min intervals which were examined with analyses of variance. Rates were consistent within each rat, although inter-animal variability was pronounced. Unstimulated transport involved minimal amounts of mucus and averaged 4.5 +/- 1.4 mm/min. Pilocarpine, presumably a secretagogue, caused a sustained (55 +/- 10 min) increase to 5.7 +/- 1.6 mm/min. Isoproterenol, presumably a cilioexcitatory agent, caused a brief (14 +/- 6 min) peak in transport at 6.4 +/- 2.1 mm/min. There were slow and fast animals which remained relatively slow or fast during and after treatment, indicating that each rat had an individual base-line determined by factors other than those susceptible to stimulation. Transport ceased almost at once throughout the airway, as mucus became scanty although ciliary activity was wide-spread. It was concluded that both types of response revealed different mechanisms for the physiological modulation of mucociliary transport.