TRH and a new TRH analog (all L-pyro-2-aminoadipyl-histidyl-thiazolidine-4-carboxamide, MK-771) have been compared with several other peptides for their "analeptic" activity and their ability to enhance the excitatory actions of microiontophoretically applied acetylcholine (ACh) on cerebral cortical neurons of rats. TRH and MK-771 offset the narcosis induced by pentobarbital in mice, whereas the C-terminal free acid derived from TRH, melanostatin, somatostatin and pyroglutamyl-histidineamide have been found inactive. Similarly, of these peptides only TRH and MK-771 induced a tremor of the forepaws in pentobarbital-anesthetized mice. Employing comparable ejection currents and durations, only TRH and MK-771, applied by microiontophoresis, enhanced the excitatory actions of ACh on spontaneously active cortical neurons in anesthetized rats. Based on these findings and other recent data, it is suggested that the interactions of TRH and MK-771 with cholinergic mechanisms may underlie some of the actions, including their anti-anesthetic effects, of these peptides.