To determine whether neuronal histamine influences development of histamine H-1 receptors in the rat brain, neonates were given diphenhydramine, an H-1 antagonist, daily for the first 21 days of postnatal life. In control rats, specific H-1 binding of [3H]mepyramine in whole brain was low at birth and increased progressively toward adult levels by the end of the 3rd week. Animals treated with diphenhydramine showed marked elevations in binding as early as 4 days postnatally and the differences persisted throughout the experimental period. The increased binding in treated rats was specific to treatment with H-1 antagonists (diphenhydramine, mepyramine) as opposed to an H-2 antagonist (cimetidine); the alteration reflected enhancement of the number of binding sites without a change in Kd, a finding indicative of H-1 receptor supersensitivity. Similar results were seen in the hypothalamus, an area rich in neuronal histaminergic projections. Levels of histamine in the hypothalamus were unaffected by diphenhydramine administration until the end of the 3rd week, at which time, increases were noted. After the loading of the hypothalamus with [3H]histamine, release of label by K+ depolarization in vitro (an indirect index of release from the neuronal histamine pool) was readily obtained at all ages and was unaffected by diphenhydramine administration. Diphenhydramine did not alter histamine levels in brain regions in which most of the histamine is non-neuronal. These studies suggest that, even early in the ontogenesis of histaminergic synapses, a sufficient proportion of releasable histamine is associated with histaminergic neurons to enable trans-synaptic modulation of development of histaminergic H-1 receptors; consequently, blockade of these receptors with diphenhydramine results in supersensitivity even at the earliest stages of development.