The ontogeny of [3H]kainic acid binding in rat forebrain was studied quantitatively using in vitro receptor autoradiography. Specific binding was detectable in ventral thalamus, hippocampus, striatum and olfactory bulb by postnatal day 1. In regions with high densities of receptors in adulthood, such as CA3, dentate gyrus and striatum, binding increased progressively across development peaking at postnatal day 21. In ventral thalamus and the inner lamina of the neocortex, [3H]kainic acid binding was high in the first three postnatal weeks and relatively low thereafter. Saturation studies performed on adults and 14-day-old animals suggest differences in both the affinity and the maximal binding capacity contributed to the observed developmental changes in binding of [3H]kainic acid.