We investigated whether and how the timing of musical rhythms changes with tempo. Twelve skilled pianists played a monophonic eight-bar melody in 21 different rhythmic versions at four different tempi. Within bars, the rhythms represented two isochronous patterns and all possible ordered pairs and triplets of different note values with ratios from the set {3, 2, 1}. The three-note rhythms also occurred in each of two meters (3/4 and 6/8). Significant deviations from the notated interval ratios were observed in performances of most rhythms, even at the slowest tempo. The observed ratios of the two-note rhythms changed little with tempo. By contrast, those of the three-note rhythms showed increasing assimilation of the two longer intervals as tempo increased, while the relative duration of the short interval was barely affected by tempo. These results replicate previous findings of Fraisse (1956), obtained in a nonmetrical and non-musical context. At fast tempi, the distinction between three different interval durations seems difficult to maintain.