Specific binding of [3H]SCH 23390 to dopamine D1 receptors in the striatum and olfactory tubercle in 14-day alloxan-induced diabetic rats was investigated. The Scatchard analysis revealed decreased D1 receptor density in the striatum (Bmax values were 548 ± 23 fmol/mg protein for the control and 466 ± 33 fmol/mg for the diabetic rats). No change was observed in the olfactory tubercle (Bmax; 299 ±27 fmol/mg for the control and 317 ± 32 fmo/mg for the diabetic rats). Thus, specific binding of [3H[SCH 23390 to striatal and olfactory tubercle membranes showed region-specific changes of brain dopamine D1 receptors in alloxan diabetic rats.