Rous sarcoma virus (RSV) RNA leader contains three short upstream open reading frames. We have shown recently that bothuORFs 1 and 3 Influence in vivo translation of the downstream gag gene and are involved in the virus RNA packaging process. In this report, we have studied the translational events occurlng at the upstream AUGs in vivo. We show that (i) the first and third AUGs are efficient translational initiation sites; (ii) ribosomes reinitiate efficiently at AUG3; and (iii) deletions in the intercistronlc distance between uORF1 and 3 (which is well conserved among avian strains) prevent ribosome Initiation at AUG3, thus Increasing translation efficiency at the downstream AUG gag. The roles of the uORFs in translation and packaging are discussed.