The study is focused on Estonian rhythmic structure as revealed in fluent read speech. The core of the study involves determining the distinctive features of the three degrees of Estonian phonetic quantity and assessment of the significance of those features by statistical methods with an aim to enhance the naturalness of synthetic speech by using the features best identifying each quantity degree in fluent Estonian speech. The theory of adjacent phones is tested on a large data set and the role of intensity as a possible feature to identify quantity degrees is investigated. According to the results of phonetic and statistical analysis the main constitutive factors of quantity degrees and, thus, of speech rhythm are the classical duration ratio of stressed and unstressed syllables, whereas the rest of the duration ratios and tonal characteristics investigated turned out to be less significant for the data analysed.