Normal mice (+/+) and homozygous weaver mutant mice (wv/wv) at 1 year of age were used for three-dimensional computer-aided reconstructions of the nucleus accumbens (NA) and neostriatum (ST) and for quantitative estimations of the total number of medium-sized neurons in the NA and ST, and for the total number of tyrosine hydroxylase (TH)-containing neurons in the ventral tegmental area (VTA) and substantia nigra (SN). The three-dimensional reconstructions showed that the weaver NA and ST are smaller than they are in +/+. Quantitative volumetric measurements of the NA and ST showed wv/wv were smaller than +/+ by nonsignificant differences of 14% and 13%, respectively. The wv/wv group showed statistically significant depletion of neurons in all four structures. On average, NA neurons are reduced by 27%, ST neurons by 22%, VTA-TH neurons by 40%, and SN-TH neurons by 79%. In wv/wv animals, there was a high positive correlation (r = 0.836) between the numbers of SN-TH neurons and ST neurons and a moderate positive correlation (r = 0.534) between the numbers of SN-VTA neurons and NA neurons. The nuclei in TH- containing neurons in wv/wv and +/+ had the same diameters, but in all animals, the SN-TH neurons contained larger nuclei than the VTA-TH neurons. Cytoarchitectonic measurements in control and weaver NA and ST were also similar. In all animals, the NA contains more densely packed neurons with smaller nuclei than those in the ST.