The specific binding in rat brain in vivo of [125I]HEAT ([125I]iodo-2-[beta-(4-hydroxyphenyl)ethylamino methyl), a selective alpha 1-adrenoceptor ligand, was analyzed by a method designed to distinguish sites with different affinities. The data indicate at least two sites with different affinities for the alpha 1-adrenoreceptor in normal rat brain in vivo: a high-affinity site with Kd (half-saturation constant) of 3.6 +/- 0.7 nM (AV +/- SD), and a low-affinity site with Kd of 668 +/- 552 nM. The density (Bmax) of the high-affinity site in nine brain regions--auditory, visual, sensorimotor (four layers) and frontal cortex and lateral thalamic and medial geniculate nuclei--varied from 2.2 +/- 0.8 to 14.6 +/- 0.6 pmole/g, while the low-affinity range was 149 +/- 44 to 577 +/- 30 pmole/g. The results also revealed a very dynamic relationship between the two sites regulated by the concentration of the ligand ranging from 80% preponderance of the high-affinity sites at low ligand concentrations to more than 95% preponderance of the low-affinity sites at high ligand concentrations.