Pure Substance P was applied from 7 mM solutions by microiontophoresis to cuneate neurones in cats under barbiturate or inhalation anesthesia. The predominant effect was a slow excitation, beginning after a delay of 10–30 s, reaching a peak some 30 s later, and decreasing only gradually after the end of the application, over 1 min or even longer. It was observed with about half of all the units tested (especially those that were spontaneously active), in all experiments, with several different electrodes, and Substance P obtained from two different sources; and therefore can be considered to reflect a genuine strong but slow depolarizing action. Larger doses of Substance P depressed firing, especially when evoked by glutamate, and, in one experiment, several units showed a mainly depressant effect. This may be explained by a similar slow mechanism of depolarization but with inactivation being predominant. It is concluded that Substance P is unlikely to be the quickly acting transmitter released by primary afferent terminals but its strong excitatory action may be of functional significance in some other respects.