Alterations in responsiveness to stimulation of G protein-coupled receptors in the cardiovascular system have been noted during aging and may be due to changes in the G proteins. The levels of G protein alpha subunits and their mRNAs were examined in the aorta and heart of adult (6-month-old) and senescent (24-month-old) Fischer 344 rats. Western blot analysis using anti-Gs alpha antiserum revealed 4 bands in the aorta (38, 42, 45, and 52 kD). The 42 kD band decreased significantly in 24-month-old rats, whereas the other bands did not change. Western blots using anti-Go alpha and anti-Gi alpha revealed single bands of 39 kD and 41 kD respectively, which decreased significantly in the aortas from 24-month-old rats. There were no significant changes in the levels of G alpha mRNAs in the aorta during aging. In the heart, anti-Gs alpha antiserum detected 3 bands (42, 45, and 52 kD) which did not change with age. Levels of Gi alpha (41 kD) and Go alpha (39 kD) also did not change with age in the heart. The mRNAs for Gi2 alpha and Gi3 alpha were significantly increased in the heart, but there was no change in Go alpha or Gs alpha mRNAs. Reduced expression of Gi alpha may provide the molecular basis for alterations in receptor responsiveness in the aging vasculature. Altered expression of G protein alpha subunits does not appear to account for age-related alterations in cardiac function.