Cardiac overload was studied in rats by abdominal aortic constriction, which increased the left ventricular weight by 59% after 12 days. During the transition, which precedes the compensatory hypertrophy, both total RNA and poly(A)-containing messenger RNA increased. The concentration of these polynucleotides peaked by day 4 after constriction, rising from 1.27 +/- 0.3 to 1.88 +/- 0.2 mg g-1 fresh weight for total RNA, and from 38 +/- 24 to 62 +/- 12 micrograms g-1 fresh weight for poly(A)-containing RNA, and returned to normal by day 12. However, the total amount per ventricle of both RNAs remained high. Poly(A)-containing RNA prepared from normal heart was hybridized to its cDNA copy. These results were expressed as the percentage of hybridization vs. the log 10 of the product of the poly(A)-containing RNA concentration and the time (Rot), and in computer analysis were described by division into three different frequency components. In normal hearts, the Rot 1/2 values of these components were, respectively, 3.98 X 10(-3), 0.338 and 21.380 mol.s.1(-1), which correspond to 2-3, 240 and 12,200 different sequences that were copied 22,000-33,000, 310 and 5 times, respectively. Four and 30 days after banding there was a harmonious enhancement of the number of the copies without any change in the number of different sequences, and the three different hybridization curves were superimposed. In conclusion, cardiac overload raises the poly(A)-containing RNA concentration, probably by stimulating transcription, but no major changes occur in any of the frequency classes.