The existence of three distinct types of proton-gated sodium currents classified in accordance with their kinetics of desensitization as fast, medium and slow, has been confirmed in the present study on isolated rat trigeminal ganglion neurons. The emphasis was put on the investigation of a peculiar medium type of a chemically gated channel population, using the concentration jump method. The features of the medium-type mechanism allow to distinguish it from the other already known types: it was insensitive to the Ca antagonist verapamil (in contrast to proton-gated sodium current found in dorsal root ganglion neurons), displayed a strong dependence of the kinetics of desensitization on the membrane potential, and (besides the apparent proton-gating) was activated and desensitized by the application of ammonia containing solution at normal pH values. The effect of ammonia itself appeared to be a good tool for the separation of fast and slow proton-gated responses. The results obtained allow to postulate a nonspecific proton-activation of medium-type receptor-channel complexes and their specificity to ammonia (A'< j= 10-4 mol/1) as an agonist.