The distribution of [3H]substance P ([3H]SP) binding sites in the brainstem of the human newborn was investigated in eleven cases (aged 1 h to 6 months) by in vitro quantitative receptor autoradiography. The binding of [3H]SP to newborn brainstem tissue was found to be saturable (for the eight cases examined, Kd and Bmax (M +/- S.E.M.) were 0.29 +/- 0.03 nM and 206 +/- 21 fmol/mg tissue, respectively). Competition studies showed unlabeled SP to be the most potent peptide for displacing [3H]SP binding from tissue sections. The desaturating effect of GTP on the specific binding of [3H]SP was also investigated, but was not found to be significant. Autoradiographic analysis showed that the neurokinin-1 (NK-1)/SP binding sites were widely but unevenly distributed, and that they varied with age. The highest densities of (NK-1)/SP binding sites were observed in the locus coeruleus, olivaris inferior nuclei, raphe magnus and obscurus nuclei, while low to moderate densities were observed in other brainstem structures. These findings support the idea that SP is involved in cardiovascular regulation, and that it may interact with the catecholaminergic and/or serotonergic system.