In beginning of muscle development, determination is induced in the mesoderm, and then differentiation occurs with accumulation of muscle structural proteins. Mesoderm cells differentiate to many type cells, but the direct signaling activator for muscle determination is still unknown. In this paper we report some of the conditions required for determination of muscle. Muscle determination during Xenopus development was found to be marked by Xmyf-5 and XmyoD expression, but not by expression of Xmyogenin or Xmrf4. Xmyf-5 and XmyoD expression was first detected in the early gastrula stage. Xmyf-5 expression was first detected on the dorsal side, whereas XmyoD was initially expressed on the ventral side. Subsequently, expression of both genes was strongly induced on the dorsal side. The expression of Xmyf-5 and XmyoD did not continue to increase on the ventral side when it was separated from the dorsal side, although muscle originates from the both sides. These findings suggest that a continuous increase in expression of both genes require the dorsalizing signal. The mesoderm inducers bFGF and Activin A induced both genes in animal caps, and the inductive activity of Activin A was stronger than that of bFGF. Overexpression of Xbra, a pan-mesoderm marker, alone induced both genes, but weakly. The inductive activity of Xbra was enhanced by co-injection with noggin. This suggests that inhibition of BMP4 by noggin in the mesoderm mediates dorsalizing signal, and may induce the direct dorsalizing activator genes of Xmyf-5 and XmyoD.