Experiments in infant rats have revealed patterns of intense acetylcholinesterase (AChE) activity in primary sensory thalamic nuclei and thalamocortical recipient zones of primary sensory areas of cerebral cortex. The distribution and appearance of AChE staining indicates that the transient AChE in these areas is characteristic of thalamocortical neurons. The patterns of AChE are detectable during the first postnatal week of life, reach peak intensity during the second postnatal week, and decline to normal adult levels by the end of the third postnatal week. These temporal parameters correlate well with the time of thalamocortical axon growth into the cerebral cortex. These data suggest that the transient AChE may be playing a morphogenic role in the development of thalamocortical connections.