The hyperfunctioning dopamine hypothesis in the mesocorticolimbic (MCL) system has been addressed by a neurogenetic approach in model systems. Thus, a morphometric analysis was carried out on neurons of origin of Substantia Nigra (SN) and Ventral Tegmental Area (VTA) dopamine systems of the Naples High-Excitability (NHE), Low-Excitability (NLE) and control lines. Male adult rats were tested in a spatial novelty for indices of activity and non-selective attention. Mesencephalic coronal sections were processed for tyrosine hydroxylase (TH) immunohistochemistry and cytochromoxidase (C.O.) histochemistry. Image analysis in the rostro-caudal plane showed (i) a higher neuron size of TH+ elements in the VTA of NHE and NLE, across the entire structure in the NHE, and only in the middle portion in the NLE; (ii) a higher expression of TH in the neuropil of the VTA in NHE; (iii) a lower C.O. activity in both NLE and NHE; (iv) no differences in the SN. The larger neuron size in both NHE and NLE rats as compared with control rats, along with higher TH expression mainly in the NHE, in absence of any relevant alteration in the SN, reveals an unbalance between the two dopamine systems and a subsequent alteration in limbic (reward, motivation, sustained attention) functions. The decreased C.O. activity might be due to reduced feedback inhibition by striatal GABA neurons and interneurons leading to increased DA neuron firing. In conclusion, the increased behavioral activity and impaired attention observed in the NHE rats are associated to hyperfunctioning MCL system in this genetic model of Attention-Deficit Hyperactivity Disorder (ADHD).