Alternative RNA processing of the calcitonin/CGRP gene generates transcripts encoding predominantly calcitonin in thyroid C cells or CGRP in the nervous system. To examine the RNA processing choice of this gene in a wide variety of tissues, we created transgenic mice expressing the rat calcitonin/CGRP transcript from the mouse metallothionein-I promoter. Most cells that do not express the endogenous calcitonin/CGRP gene have the capability to make a clear splicing choice for calcitonin or CGRP transcript. In the majority of tissues studied, 90%-97% of the transgene mRNA encodes calcitonin. In contrast, both calcitonin and CGRP mRNAs were detected in the transgenic mice brains. Immunohistochemical and in situ RNA hybridization analyses show that CGRP transcripts are selectively expressed in a wide variety of neurons, while calcitonin is expressed predominantly in nonneuronal structures. Splicing choice operates independently of calcitonin/CGRP gene transcription. The data suggest that a specific regulatory machinery is required for the processing of CGRP transcripts and is restricted primarily to neurons.