Abstract Cardiac responsiveness to β-adrenergic stimulation changes with age. Developmental changes in expression of guanine nucleotide-binding coupling protein (G protein) subunits may account for these physiologic changes, We measured steady-state levels of mRNA encoding the α-subunit of the specific G protein that stimulates adenylyl cyclase (G sα) and three isoforms of β-subunit of G proteins (Gβ) in developing myocardium. Total RNA prepared from the right and left ventricles of fetal, neonatal, juvenile, and adult rabbits was size-fractionated, blotted, and probed with 32P-labeled cDNAs encoding rat G sα, bovine Gβ-1, human Gβ-2, and human Gβ-3, For standardization, these blots were subsequently hybridized with a 32P-labeled cDNA encoding glyceraldehyde 3-phosphate dehydrogenase (GAPD). Two-dimensional densitometric analysis of autoradiographs was used to quantify relative hybridization intensities. An age-dependent decrease in mRNAs encoding G sα, Gβ-1, and Gβ-2 relative to mRNA encoding GAPD was observed in both ventricles, while Gβ-3 mRNA was not detected. At all ages studied, levels of G sα and Gβ-1 mRNA were similar in the two ventricles. However, Gβ-2 mRNA declined more in the left ventricle than in the right ventricle during maturation. Our results demonstrate developmental control in heart for mRNAs encoding several G protein subunits, In addition, differential declines in Gβ-1 and Gβ-2 mRNA in the right ventricle suggest that these Gβ isoforms are regulated uniquely and may reflect functional roles for these Gβ isoforms in different signaling cascades.