It is generally admitted that the sound sequence ^*μγα(*ü ge ) has developed in Khalkha Mongolian into the two long vowels: o (ö) and a (e). Different explanations have been given as to the cause of this divergent development. Vladimirtsov regarded the contraction *μγα>a (*üge>e) as a general development and *μγα>o (*üge>ö) as the result of progressive labial attraction. This, however, cannot be applied to such Khalkha words as dζolo «rein», dζoro «amble» etc. which originally had the vowel *i of the first syllable. Because, if we take into consideration the Ordus forms dli15 «rein» and dzilo «amble», it is most brobable that the first vowel of Khalkha dzolo and dzcoro has been brought about by the breaking of *i into o under the influence of the following o. On the other hand, Poppe and Thomsen considered the con-traction*μγα>o (*üge>ö) to be a general development, which is, in its turn, inconsistent with other vowel constractions in Mongolian: e. g.*αγα>a, *αγu>u, *uγu>u, *oγα>o etc. As a result, they had either to take *uγa> a (*üge >e) for an ex-ception to the sound change or to assume a roundabout develop-ment such as *αburad>*abod>awid «having taken». The present author assumes at the early stage of development the sound change * uγa>*oγa (*üge>*öge) after the vowels *o (*ö) and *i of the first syllable, only by way of which we can accept the followings as regular sound changes in Mongolian: 1. the vowel contractions *uγa>a, *üge>e, *oγa>o, *öge>ö, 2. the breaking of *i into o (ö) before o (ö). The long vowel a (e) in Kalmuck (e. g. dzola «rein», dlora «amble») should be explained either by the relaxation of the rounded vowel o (ö) or by the contraction *oγa>a (*öge>e) peculiar to this dialect.