In Brazilian Portuguese, there are at least two types of unstressed syllables: pretonic and final. Phonologically, this is shown by the fact that the system of 7 phonemic oral vowels in stressed syllables is decreased to 5 in pretonic syllables and to 3 in unstressed final syllables. This paper examines the changes in vowel quality and quantity which occur as the number of contrasts decreases. Spectrographic analysis of formant frequencies and durations for 4 male speakers show that centralization is greater and durations shorter finally than in pretonic position.