This thesis describes studies on PK-A activity and the expression of generic C- and R-subunits in the rat mammary gland throughout the pregnancy/lactation/involution cycle. "Expressed" and "total" activities of PK-A were measured in extracts of rat mammary tissue sampled throughout pregnancy and lactation. Expression of the genes encoding the C-subunit isoforms Cα and Cβ was examined by northern blotting, as a function of mammary development, to determine relative levels of their respective mRNAs. The content of C-subunit protein (all isoforms) was also estimated immunochemically and related to levels of C-subunit catalytic activity and of mRNAs. It was found that C-subunit isoform mRNAs are expressed coordinately during mammary development and that a marked decline in expression, per cell, at around parturition is paralleled by a fall in total PK-A activity. The expressed activity of PK-A underwent characteristic changes throughout pregnancy and lactation, reaching a peak late into pregnancy. The PK-A activity ratio reached a peak in early lactation. C-subunit protein mass paralleled total PK-A activity throughout pregnancy and lactation, thereby demonstrating the constancy of C-subunit specific activity during these developmental events. Regulatory subunits (R-subunits) were determined by means of a photoaffinity labelling assay, using 8-azido [³²P]cAMP; both particulate and cytosolic fractions of mammary tissue were examined. The type I isoform (RI) best reflected changes in the rate of cellular proliferation in the tissue. The abundance of RII as a proportion of total R-subunit increased throughout pregnancy and lactation in both cellular compartments. Litters were removed from mid-lactating rats, interrupting the differentiated function of the mammary tissue. Accompanying this, total and expressed PK-A activities were diminished after 24 and 48h; C-subunit mass was also decreased but not in proportion to total activity such that specific activity was also reduced. These effects were apparently not transcriptionally driven, since the levels of Cα mRNA remained unaltered.