To determine if the expression of the epsilon-subunit of the acetylcholine receptor by the subsynaptic nuclei in skeletal muscle is dependent on the state of differentiation of the muscle, we have compared the spatiotemporal distribution of epsilon-subunit transcripts during synapse formation in fetal and adult muscle. Both during "ontogenic" synaptogenesis in the fetus and during "ectopic" synaptogenesis in the adult animal the motor nerve induced focally the expression of the epsilon-subunit mRNA in subsynaptic nuclei. The temporal expression patterns at both types of developing synapses were similar. The results support the view that in muscle developing in vivo epsilon-subunit gene transcription and its stabilization in subsynaptic nuclei is exclusively controlled by the motor neuron, independently of the developmental state of the muscle nuclei. Thus, both nerve and muscle remain plastic in their respective abilities to induce and express the synapse-specific combination of AChR subunit genes.