Abstract: To study potential biochemical correlates of dopamine (DA) and serotonin receptor supersensitivity, rats were lesioned at 3 days after birth with 6‐hydroxydopamine (6‐OHDA; 67 µg in each lateral ventricle; desipramine pretreatment, 20 mg/kg i.p., 1 h) and then sensitized with the DA D1 agonist, SKF 38393 HCl (3.0 mg/kg i.p. per day) either ontogenetically (daily, for 28 consecutive days from birth) and/or in adulthood (four weekly injections, 6–9 weeks from birth). Controls received vehicle in place of 6‐OHDA or SKF 38393. Enhanced locomotor responses were observed after SKF 38393 at 6 weeks, only in rats that received SKF 38393 + 6‐OHDA in ontogeny. Locomotor responses were further enhanced in this group after the last of four weekly SKF 38393 injections at the 9th week. These weekly SKF 38393 treatments also produced enhanced responses in 6‐OHDA rats that did not receive SKF 38393 in ontogeny. When striata were studied at 11 weeks, the percentages of high and low affinity DA D1 binding sites were not altered. Basal as well as DA‐, NaF‐, and forskolin‐stimulated adenylyl cyclase activities also were not changed. Dot blot analysis showed that there was a reduction of mRNA levels for DA D1, but not serotonin1C, receptors in the 6‐OHDA groups. However, SKF 38393 at 6–9 weeks eliminated this alteration. Based on these findings it can be proposed that supersensitization may be a consequence of altered neuronal cross talk rather than an imbalance of receptor elements per se.