Use of prometaphase chromosome preparations has led to significant improvements in the localization of both NORs and ribosomal gene clusters in the short arms of human acrocentric chromosomes. An improvement of the NOR silver-staining method, followed by trypsin-Giemsa banding, was used to identify the precise location of the NOR on each human acrocentric chromosome. For comparison, the satellite, stalk, and centromeric region were also identified with the aid of both Q- and G-banding techniques. The amount of silver impregnation present in the stalk region of the D- and G-group chromosomes was unique for each of these acrocentric chromosomes and depended on the length of the stalk. In situ hybridization was used to locate the ribosomal gene clusters. A plasmid containing 5.6 kb of the 18S rDNA gene was first oligolabeled with bio-16-dUTP, then hybridized in situ to metaphase chromosomes and visualized by an alkaline phospha-tase color-detection system. Our results indicated that, in most cases, the location of the 18S rDNA gene cluster in the stalk region was indistinguishable from the site of silver impregnation. However, exceptions were noted, suggesting a multiplicity of arrangements of the ribosomal gene clusters. A model is proposed to describe the spatial relationship of NORs (transcriptional activity) and the ribosomal gene clusters.