Abstract: To study speech mimicry in hill mynahs Gracula religiosa is one of the channels to understand vocal learning in birds. Experiments were designed in order to study the critical period and social interaction affecting this learning behaviour. The variations of learning ability depended upon subspecies, sexes and sex hormone levels were also investigated. The studies were divided into five experiments. The results showed that the hand-reared nestlings in individual cages, after learning Thai words and phrases before six months of age for a year, were the most effective group to show vocal imitation (18 times/h). Meanwhile, wild birds older than six months, though reared and trained in individual cages, did not produce vocal imitation. The variations of vocal imitation between subspecies and sexes were not found. However, male birds with high vocal imitation efficiency (15 times/h) had higher testosterone levels than those with lower vocal imitation efficiency. Estradiol levels were high in female birds with high vocal imitation. The critical period of vocal imitation was the first six months of age and social contact with a trainer was required, without interacting with other hill mynahs. It seems likely that endocrine states, social interactions and vocal imitation output are correlated.