The ratios of calcitonin (CT) to calcitonin gene-related peptide (CGRP) mRNA, both generated by alternative RNA processing from the same primary RNA transcript, are shown by Northern blotting of cytoplasmic RNA to vary as a function of growth in a human medullary thyroid carcinoma cell line (TT). Upon initial seeding, CT mRNA levels are relatively high, and CGRP mRNA levels are relatively low. During the early logarithmic growth phase, CGRP mRNA levels rise severalfold, while CT mRNA levels change only slightly. As the cells approach confluence, both CT and CGRP mRNA levels rise. Subsequently, CGRP mRNA levels fall substantially in postconfluent cells, while CT mRNA levels remain high. By actinomycin D blocking of nascent transcription, we have shown that these growth-related, reversible changes in the ratio of CT to CGRP mRNA are not due to changes in mRNA stability. Our data rather suggest that TT cells reversibly alter alternative RNA-processing patterns dependent upon growth conditions in vitro, such that CT mRNA is lowest and CGRP mRNA is highest during rapid growth. The mechanisms underlying this RNA-processing alteration may play a role in certain patients with aggressive forms of medullary thyroid carcinoma, in whom a decrease or loss of CT levels heralds a poor prognosis.