Alternative hypotheses regarding the vowel system of Proto-Sahaptian and the transition to the two daughter languages, Nez Perce and Sahaptin, are examined. The proto-language may possibly have had a five-vowel system ∗/i e a o u/, identical to that of Nez Perce, and already showing the same pattern of vowel harmony alternations. But a reconstruction of the vocalic system before the development of vowel harmony, whether or not this predated Proto-Sahaptian, presents several possibilities. A six-vowel system ∗/i ə e a o u/ would allow a straightforward development. A suggested three-vowel system is rejected. The possibilities of development from four- or five-vowel systems are examined, but these, while conceivable, are less plausible. Examination of the comparative evidence suggests that Proto-Sahaptian possessed the six-vowel system and may have lacked vowel harmony. Some areal similarities to such a system are noted.