Procalcitonin (PCT) is a highly sensitive and specific marker of systemic bacterial infection and sepsis. In contrast to its diagnostic significance, the cellular sources of plasma procalcitonin remain to be clarified. Two forms of PCT mRNAs originate from calcitonin/calcitonin gene-related peptide gene (CALC-I gene) along with mRNA for calcitonin gene-related peptide-I (CGRP-I). Reverse transcription polymerase chain reaction with newly designed primers detecting different PCT mRNAs and CGRP-I mRNA was used to identify tissues that might contribute to PCT production. Our study indicates that a variety of human tissues (13 of the 16 analyzed overall) express PCT-I, PCT-II, and/or CGRP-I mRNAs, with the highest levels detected for liver, testis, lung, prostate, kidney, and small intestine. Various tissues differ in the proportions of PCT-I, PCT-II, and CGRP-I mRNA expression levels. Thus we demonstrate the complexity of tissue-specific regulation of CALC-I gene expression and suppose a variety of tissues as a potential source of CALC-I-encoded peptides.