Polychlorinated biphenyls (PCBs) elicit endocrine disruptive effects in many species, including birds. Tree swallows (Tachycineta bicolor) were studied at eight sites, located in Maryland, Pennsylvania, and New York, with a range of PCB contamination to determine effects on gender and gonadal development of nestling offipring. Blood samples were collected from nestlings and genetic sex was determined by polymerase chain reaction amplification of sex chromatin in nucleated red blood cells. Gonads were excised and fixed for subsequent gross and histologic examination. PCB analyses of twelve-day old nestlings indicated that residue concentrations varied considerably among the eight sites. Of the 145 nestlings examined anatomically, the phenotypic sex ratio was 53% female and 47% male. No intersexes were observed. Histological observations revealed some variation such as numbers of spermatogonia and stages of follicular development among individuals. Genotypic evaluation of the 145 nestlings revealed complete concordance with phenotypic observations. Although there were significant differences in PCB exposure among study sites, there was no evidence of abnormal gonadal development or anatomical gender alteration in nestling Tree swallows.