Extracellular recordings from raphe dorsalis neurons in rats during development were performed, and the response to iontophoretically applied beta-antagonists was analysed. In pups younger than 2 weeks, raphe dorsalis firing was totally or partially inhibited by application of beta-antagonists, while after 2 weeks of age the neurons were insensitive to the same application. The beta-receptors appeared to be involved in the control of raphe firing during a transient period of ontogenesis and thus the disappearance of beta-regulation at older ages may reflect the regression of this population of neurotransmitter receptors.