We investigate the perception and production of Czech /I/ and /i:/, a contrast traditionally described as quantitative. First, we show that the spectral difference between the vowels is for many Czechs as strong a cue as (or even stronger than) duration. Second, we test the hypothesis that this shift towards vowel quality as a perceptual cue for this contrast resulted in weakening of the durational differentiation in production. Our measurements confirm this: members of the /I/-/i:/ pair differed in duration much less than those of other short-long pairs. We interpret these findings in terms of Lindblom's H&H theory.