This study investigated the associated effects of exposure to current-use orchard pesticides and p,p'-DDE on concentrations of corticosterone in response to stress, circulating thyroid hormone levels and thyroid histology, the response to a phytohaemagglutinin (PHA) skin test as well as immune tissue histopathology in two species of passerines. Wild 16-day-old tree swallows ('Tachycineta bicolor') and 12-day-old eastern bluebirds ('Sialia sialis') from four apple orchards sprayed with pesticides and three non-sprayed reference sites in southern Ontario were tested during 2000-2001. Nestling birds were exposed to as many as seven individual pesticide applications and up to five mixtures of pesticide sprays. Concentrations of p,p'-DDE in tree swallow eggs ranged from 0.054 to 5.440 mg/kg (wet weight); bluebird eggs contained levels from 0.167 to 95.381 mg/kg (wet weight). Following a standardized handling stress coupled with adrenocorticotropic hormone (ACTH) challenge, tree swallow nestlings from sprayed orchards showed significantly higher levels of corticosterone relative to non-sprayed chicks; while bluebird chicks exhibited a significant negative association between circulating corticosterone and increasing concentration of p,p' -DDE found in individual egg samples. (Abstract shortened by UMI.)