The ontogenetic pattern of multiple opioid binding sites in rat brain from birth until weaning has been investigated. [3H]-dihydromorphine ([3H]-DHM)3 [3H]-D-Ala2-D-Leu5-enkephalin ([3H]-DADLE) and [3H]-dynorphin A (1-8) ([3H]-DYN) as markers of mu (mu), delta (delta) and Kappa (kappa) sites were utilized respectively. The analysis of the kinetic parameters of [3H]-DHM binding shows that, at birth, mu sites possess an affinity similar to that of adult animals, and a density of 50%, which reaches 80% of the adult value at day 4. On the contrary, [3H]-DADLE binding in the first post-natal days shows low affinity and low density and delta-sites do not reach values comparable to the adult ones until the second week of life. The kinetic parameters of [3H]-DYN binding are almost undetectable during the preweanling period, due to the very low density of kappa sites at this stage of life. Displacement studies with mu-, delta- and kappa-selective ligands show that the Ki values on [3H]-DHM binding sites were similar in 4 day old and adult animals for all the tested compounds, whereas Ki values on [3H]-DADLE and [3H]-DYN binding sites reflected an immaturity of delta and kappa receptors. In conclusion, our data suggest that multiple opioid receptors follow different ontogenetic patterns. In the first stages of life only mu receptors are almost mature and possibly mediate endogenous opioid actions and exogenous opiate pharmaco-toxicological effects.