The extracellular concentration of inositol 1,4,5-trisphosphate (IP3) has been monitored in the ventral hippocampus of the anesthetized rat by using a microdialysis technique coupled to a radioreceptor assay. Three hours after the implantation of the cannula, basal extracellular concentration of IP3 (corrected for a 9% recovery) was 71 nM (0.39 pmol/60-microliters fraction) and remained stable for at least 5 h. Local infusion of carbachol for 60 min caused a significant concentration-related increase in extracellular IP3 levels (0, 24, and 57% at 1, 50, and 100 microM, respectively). Acetylcholine (100 microM) and muscarine (100 microM) increased IP3 outflow by 40 and 42%, respectively. The effect of carbachol was fully prevented by coinfusion of 10 microM pirenzepine and reduced by 1 microM tetrodotoxin indicating that the carbachol response is mediated by neuronal muscarinic receptors. These data demonstrate the feasibility of using microdialysis and a radioreceptor assay to measure IP3 in the extracellular space. This approach could prove useful for the study of the in vivo operation of muscarinic and, by extension, a number of receptors coupled to phosphoinositide turnover.