We tested the hypothesis that exposure of chick embryos to corticosterone leads to increased fear, reduced competitive ability, reduced ability to cross a barrier and reduced growth in juvenile chicks. Behaviour was studied in birds subjected to three different egg injection treatments: a negative control (no treatment of eggs), a positive control (100μl sesame oil vehicle) and a corticosterone treatment (0.6μg corticosterone in 100μl sesame oil). Eggs were injected prior to incubation and the behaviour of chicks was studied during the first 4 weeks of life. Corticosterone treatment increased fear in chicks, as indicated by greater avoidance of an observer in the home pen at 2 weeks of age (P<0.0001), reduced ability to cross a wall to access feed at 2 weeks of age (P<0.05) and reduced ability to compete for a wormlike object at 4 weeks of age (P<0.01). Treatment with corticosterone also reduced body weight at 1 week of age (P<0.003) and 4 weeks of age (P<0.04), but not at hatch (P<0.28). The sesame oil vehicle reduced fear (P<0.0001), but had no other significant effects. These results indicate that embryonic exposure to corticosterone leads to behavioural and growth deficits in chicks.