The expression of the surfactant-associated proteins in bronchiolar cells remains to be defined. We used in situ hybridization to identify sites of message expression of the surfactant-associated proteins A, B, and C (SP-A, SP-B, and SP-C) in adult and fetal rat lung. The expression of these messages by in situ hybridization was also compared with the localization of SP-A by immunocytochemistry. The localization of SP-A was used to identify type II cells and nonciliated bronchiolar epithelial (Clara) cells in these sections. The cRNA antisense probes for SP-A, SP-B, and SP-C appeared to hybridize over type II cells. Sense probes showed no localization or apparent specific hybridization. Messages for both SP-A and SP-B were also found in nonciliated bronchiolar epithelial (Clara) cells. However, no message for SP-C was observed in these cells. Clara cells from terminal to large bronchioles lacked detectable mRNA for SP-C. Expression of surfactant protein mRNAs was not detectable in type I cells, alveolar macrophages, interstitial cells, or vascular cells. Similarly, in fetal rat lung the messages for SP-A and SP-B but not SP-C were detected in bronchiolar cells. We conclude that rat Clara cells do not express SP-C mRNA, and thus SP-C can be regarded as a specific marker for rat type II cells.