Previous evidence suggests that the endogenous cannabinoid system emerges relatively early during brain development in the rat. However, the pre- and postnatal pattern of appearance of CB1 cannabinoid receptors in humans has not been analysed in detail. Furthermore, there is a complete lack of information about the functional ability of these proteins to activate signal transduction mechanisms during human development. In the present study we have explored CB1 receptor expression throughout the different areas of the developing human brain by [3H]CP55 940 autoradiography. We have also assessed CB1 functional coupling to G proteins during brain development by agonist-stimulated [35S]GTPgammaS autoradiography in the same cases. Our results indicate a significant density of cannabinoid receptors at 19 weeks' gestation in the same areas that contain these receptors in the adult human brain. Autoradiographic levels of CB1 receptors in these structures seem to increase progressively from early prenatal stages to adulthood. Interestingly, high densities of cannabinoid receptors have also been detected during prenatal development in fibre-enriched areas that are practically devoid of them in the adult brain. In parallel with these data, we have found that brain cannabinoid receptors are functionally coupled to signal transduction mechanisms from early prenatal stages. This early pattern of expression of functionally active cannabinoid receptors, along with the transient and atypical localization of these proteins in white matter areas during the prenatal stages, suggest an specific role of the endocannabinoid system in the events related to human neural development.