Despite a large body of research directed at assessing the effects of perinatal cannabinoid exposure, little is known about the development of the cannabinoid receptor. Recent advances, including the cloning of the cannabinoid receptor, have afforded us the opportunity to plot the postnatal ontogeny of the cannabinoid receptor and its mRNA in whole brain using the methods of receptor binding and RNA blot hybridization, respectively. Our results indicate that cannabinoid receptor mRNA is present at adult levels as early as postnatal day 3. The Bmax, on the other hand, increases almost fifty percent with increasing postnatal age, while the affinity does not change. The Hill coefficients for all ages studied were approximately 1. These findings suggest the possibility of a developmental progression for cannabinoid receptor development with receptor mRNA appearing first, followed by a period of rapid proliferation of the receptors themselves.