Red-winged blackbird (Agelaius phoeniceus: red-wings) and European starling (Sturnus vulgaris: starling) nestlings were dosed with either 2.0 mg/kg body weight chlorpyrifos, 50.0 mg/kg body weight dimethoate, or a propylene glycol carrier in situ. The sensitivity to these compounds was compared between species and between adults and young of the same species. In addition, four growth measurements were recorded for nestlings to determine if these organophosphorus compounds caused perturbations in development at sublethal concentrations. Red-wing nestlings were more sensitive to chlorphyrifos than starling nestlings and starling nestlings were more sensitive to dimethoate than red-wing nestlings. In addition, red-wing nestling were more tolerant of a substantially higher concentration of dimethoate than the adult LD50. The sensitivity of starling nestlings to dimethoate was similar to adults. In contrast, both species were more sensitive to chlorpyrifos than adults. After the initial 24 hr, nestlings dosed with either chemical recovered and continued their development. Of the four growth measurements, body weight appeared to be the most important in documenting pesticide-induced growth inhibition.