To elucidate the role of cAMP and different cAMP-dependent protein kinases (PKA; A-kinase) in lung cell proliferation, we investigated rat alveolar type 2 cell proliferation in relation to activation or inhibition of PKA and PKA regulatory subunits (RII-alpha and RI-alpha). Both the number of proliferating type 2 cells and the level of different regulatory subunits varied during seven days of culture. The cells exhibited a distinct peak of proliferation after 5 days of culture. This proliferation peak was preceded by a rise in RII-alpha protein level. In contrast, an inverse relationship between RI-alpha and type 2 cell proliferation was noted. Activation of PKA increased type 2 cell proliferation if given at peak RII-alpha expression. Furthermore, PKA inhibitors lowered the rate of proliferation only when a high RII level was observed. An antibody against the anchoring region of RII-alpha showed cell cycle dependent binding in contrast to antibodies against other regions, possibly related to altered binding to A-kinase anchoring protein (AKAP). Following activation of PKA, relocalisation of RII-alpha was confirmed by immunocytochemistry. In conclusion, it appears that activation of PKA II is important in regulation of alveolar type 2 cell proliferation.