The ability of mature host thalamic neurons to innervate embryonic (E19) cortex when implanted into the cortex of adult hosts was compared in normal and basal forebrain lesioned mice. The ingrowth of mature horseradish peroxidase-labeled thalamic axons into the transplants is facilitated by prior basal forebrain lesions. We discuss the possible reasons for the lesion-induced enhancement of axonal ingrowth, including the possibility that the enhanced ingrowth of thalamic fiber systems may be related to the loss of cortical innervation by extrathalamic brainstem inputs, especially cholinergic afferent fibers. The results support the interpretation that extrathalamic inputs to cortex play a modulatory role in regulating the growth and connections of specific sensory fiber systems during brain responses to injury.