Triiodothyronine (T3), injected daily into rats, induces heart hypertrophy. In a recent work we have shown that a single T3 injection stimulates RNA synthesis and modifies the translational products of myocardial mRNAs in reticulocyte lysates, quantitatively and qualitatively. In this work we show that T3 induces small but significant changes in mRNA size distribution after 4 h and much more important changes after 18 h. It also modifies the size distribution of their poly(A) tails. We studied the early effects of this compound on mRNA complexity, using the nucleic acid hybridization technique with DNA complementary to poly(A)+ RNAs. T3, 4 h after injection, suppresses approximately 15% of the sequences, mostly among rare sequences, and increases the frequency of the abundant sequences and of the sequences of intermediary abundance. A large part of this effect disappears 18 h after the injection. It may be concluded that T3 presents an early effect on gene expression, involving changes either at the gene level or and at some post transcriptional level.