Purified, cyclic GMP-dependent protein kinase (cGPK) was pressure-injected into neurons of the precruciate cortex of awake cats. Input resistances increased within seconds after injection and remained elevated for 2 min or longer. The increases were larger when cGPK was injected in a mixture with 10 microM cGMP than when injected alone. Injections of heat-inactivated cGPK, with or without 10 microM cGMP, failed to produce increases in input resistance. The present results indicate that injection of activated cGPK into neurons of the mammalian motor cortex can mimic actions of extracellularly applied acetylcholine and intracellularly applied cGMP, the latter in 100-fold higher concentrations than those used here, in neurons of the same cortical areas.