Neuron specific enolase (NSE), an isoenzyme of the glycolytic enzyme enolase, has been established by immunocytochemical means as a marker of morphological and functional maturation in central neurons and appears late in development. However, little is known about the presence of NSE in developing peripheral neurons and endocrine cells and its relationship to the development of classical neurotransmitters and peptides. We therefore investigated the appearance of NSE immunoreactivity in nerves and mucosal endocrine cells of the human respiratory tract in foetal, neonatal and adult life. NSE was found to be present in neuroblasts, nerve fibres and endocrine cells from the earliest period of gestation examined (8 weeks), before the appearance of acetylcholinesterase activity (10-12 weeks), dopamine-beta-hydroxylase (20 weeks), vasoactive intestinal polypeptide (20 weeks) or calcitonin (20 weeks). Bombesin-like immunoreactivity was found in a small proportion of mucosal endocrine cells as early as eight weeks in the foetal respiratory tract. These findings indicate that unlike central neurons and their processes, peripheral neurons of the lung contain NSE immunoreactivity well before full maturation and establishment of synaptic contact with end organs.