To function as a trophic factor PACAP and PACAP-R must be expressed in the nervous system during early development. We report here on the distribution of PACAP mRNA in the developing nervous system of the rat and compare its expression with that of PACAP-R. We discuss primary neuron culture experiments that study the neurotrophic activity of PACAP. Experimental results that indicate the presence of PACAP and its receptor in the developing nervous system, together with the observed neuropeptide activity on various populations of neurons, support the view that PACAP exhibits important neurotrophic activities comparable to those of the classical neurotrophic factors.