Human neuroblastoma cells often carry amplified DNA encompassing the gene N-myc. Amplified N-myc has been found localized in "double minutes" in direct tumor cell preparations. In contrast, later passages carried amplified N-myc almost exclusively within a single homogeneously staining chromosomal region located at a chromosomal site different from the normal location of N-myc. We used pulsed field gel electrophoresis to define the structural arrangement of the amplified DNA. Long-range mapping was facilitated by the presence of several sites for rare cutting restriction endonucleases in the 5' region of N-myc. Amplified DNAs of different neuroblastoma cell lines were heterogeneous in size and had undergone recombination at various distances from N-myc. N-myc occupied a central position within the amplified DNA, and in no case was the coding region affected by recombination. Among neuroblastoma cells, varying proportions of amplified DNA (in some instances close to 100%) consisted of multiple tandem arrays of DNA segments ranging in size from 100 to 700 kilobase pairs. Tumor cells with low degrees of amplification revealed regions of amplified DNA in excess of 1,500 kilobase pairs without apparent rearrangement. Our observations, in concert with the cytogenetic findings, suggest a model of gene amplification which involves unscheduled DNA replication, recombination, and formation of extrachromosomal DNA followed by integration into a chromosome and subsequent in situ multiplication. The central position which N-myc occupies within the amplified sequences and the lack of recombination within the coding region of N-mc indicate that N-myc rather than other genetic information provides the selective advantage for retention of the amplified DNA.