PAC1 is a recently cloned and characterized heptahelical, G protein-coupled receptor with high affinity to PACAP-27 and PACAP-38 and is differentially coupled to activate intracellular Ca2+ and cAMP. PAC1 is expressed as four major splice variants, each possessing differential coupling to inositol phosphates and intracellular Ca2+. PAC1 has been shown previously to be expressed and regulate the growth and proliferation of nonsquamous cell lung cancer cells, as well as breast cancer cell lines. PAC1 is expressed on the HCT8 human colon cancer cell line and is coupled to the activation of both intracellular cAMP and Ca2+ with consequent stimulation of growth. In the current study, we contrast the effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on the HCT8 colon cancer cell lines to the HCT116 and FET cell lines wherein PAC1 is expressed as the SV1 or HIP splice variant and is coupled to the activation only of cAMP but not of intracellular Ca2+. These data indicate that human colon tumor cells express PAC1 and are differentially coupled to intracellular signal transduction molecules. The ability to activate both cAMP and Ca2+ appears to be a prerequisite for activation of tumor proliferation, indicating a potentially important factor in how PACAP potentiates the growth of certain tumors.