We have attempted to use antisense RNA techniques in the developing Xenopus embryo and found that although hybrids form between sense and antisense RNAs, they are transient. Our studies indicate that this is due to the presence of an activity that unwinds RNA:RNA hybrids. The activity is present at low, but detectable, levels in Xenopus oocytes, increases during oocyte maturation, and exists at high levels throughout early embryogenesis. The activity has been characterized in S100 extracts. The denaturation of a specific RNA:RNA hybrid is inhibited by the addition of a second duplexed RNA but not by the addition of single-stranded RNA, single-stranded DNA, or double-stranded DNA.