The adaptability of one indigenous and four exotic breeds of pigs to the humid tropical climatic conditions of Ibadan was comparatively evaluated with acetylcholinesterase activity (AChE) of brain and hypophyseal parts as the parameter. Significant breed differences were observed in the weight and AChE of the hypophyses and in the AChE of the brain parts. With regard to sex, only the relative adenohypophyseal weight and the AChE of the cerebellum and medulla oblongata were influenced. The results demonstrate the greater adaptability of the indigenous pigs to their native environment than the exotic breeds. A reduced sensitivity to the various stressing factors of the indigenous pigs and the females was implied. Results also suggest that the Large White breed may be more suited to the conditions prevalent in Ibadan than the other exotic breeds.