K+, excitatory amino acids (EAAs) and carbachol (Carb) were tested separately or in pairs for their ability to stimulate inositol phosphate (IPs) formation in rat forebrain synaptoneurosomes. K+ ions per se, stimulate IPs synthesis (158% of the control value) as well as EAAs and Carb. The glutamate (Glu)- and quisqualate (QA)-elicited IPs formation is not additive with that evoked by K+. Inversely, K+ ions (up to 30 mM) potentiate the Carb-induced IPs accumulation. These results indicate that QA (or Glu) and Carb enhance IPs formation independently and that QA- and K+ -induced IPs responses are interdependent. This suggests that they share a 'common intermediate' step in the multistep mechanism which leads from receptor activation to the IPs synthesis. This 'common intermediate' step may be depolarization and/or Na+ influx.