1. A morphometric-statistical analysis of the subthalamic nucleus was carried out on 14 hemispheres of 12 normal human brains. 2. The values determined in paraffin embedded sections were corrected for shrinkage which showed considerable interindividual variation. 3. The mean fresh volume of the subthalamic nucleus amounted to 144 mm3 for males and 134 mm3 for females, the difference of 7% not being statistically significant. 4. In the medial part of the subthalamic nucleus the nerve cells were smaller and more closely grouped than in the lateral part which had larger nerve cells. The ratio of the volumes of the two parts was about 1:4 (medial: lateral). 5. The nucleus subthalamicus occupied 0,027% of the volume of the hemisphere. 6. The numerical nerve cell densities -- corrected for shrinkage -- were 1970 +/- 145 nerve cells/mm3 for the lateral part and 2910 +/- 310 nerve cells/mm3 for the medial part, this difference of 48 +/- 4% being highly significant. On the other hand, no difference between sexes could be shown. 7. The mean of the absolute number of nerve cells was 306000 for male and 286000 for females, the difference of 7% not being significant. 8. The volumetric nerve cell densities were found to be 1.58 +/- 0.15 vol% lateral and 1.99 +/- 0.16 vol% medial, the difference of 27% being significant. No differences between sexes were found. 9. The linear dimensions of the nerve cells of the medial part were about 10% smaller than those of the nerve cells of the lateral part. The mean fresh volume of a nerve cell was 8070 mum3 in the lateral portion and 6960 mum3 in the medial portion, the nerve cells of pars lateralis being just as large as the nerve cells of the lateral segment of the pallidum (8100 mum3). 10. The parameters determined showed good correlations with corresponding data of the pallidum. The fresh volume and the absolute numbers of nerve cells of the nucleus subthalamicus were better correlated with the pallidum laterale than with the pallidum mediale.