Abstract: A study of the onset of cation and guanine nucleotide regulation of δ, μ, and k rat brain opioid receptors during postnatal development was undertaken. Site-specific binding assays were utilized for each receptor type and the effects of 0.5 mM MnCl2, 100 mM NaCl, and/or 50 μM guanosine-5′-(β,γ-imido) triphosphate [GPP(NH)P] were assessed. The most pronounced changes of opioid binding were seen in the presence of Mn2+. In adults, agonist binding to δ sites was stimulated by Mn2+, whereas that to μ. sites was not affected and k binding was inhibited. The postnatal development of Mn2+ regulation for the three receptor subtypes was distinctly different. The largest effects were seen on δ sites detected in the early neonatal period, Mn2+ eliciting a 68% stimulation of binding over controls at day 1. Significant inhibition of k site binding by Mn2+ was detected only after the third postnatal week. Mn2+ caused a significant reversal of Gpp(NH)p inhibition of δ binding in the early neonatal period, exceeding that in the absence of regulators. Inhibition of μ and δ receptor binding by Na+ was greater, and the Mn2+ reversal of this effect was smaller, in the first 2 postnatal weeks than in adults. Gpp(NH)p + Na+ regulation did not change appreciably during the postnatal period. However, Mn2+ reversal of the considerable inhibition elicited by the combination of Na+ and Gpp(HN)p was developmental time-dependent. The data are discussed in terms of multiple sites of interaction for guanine nucleotides and cations. Our results demonstrate that the characteristics and postnatal development of guanine nucleotide and cat-ionic regulation of μ, δ, and k binding are distinctly different. Furthermore, neonates may serve as a model for the examination of individual regulatory effects on opioid receptors.