This study investigates the patterns of pre-boundary lengthening (PBL) in Czech, a language with phonemic length distinctions. Studies on Finno-Ugric languages found that PBL can be restricted or absent on vowels for which a corresponding longer phoneme with the same quality exists in the inventory. The present study provides the first investigation of a Slavic language in this regard. A production experiment was conducted that tested for an impact of vowel length on the presence and amount of PBL in trisyllabic words with antepenultimate stress. The results showed that short and long vowels undergo lengthening in all positions, but short vowels in pre-final syllables tend to involve a smaller amount of relative lengthening than long vowels. Furthermore, the results are compatible with the assumption that the initiation of PBL is linked to the stressed syllable, as has been observed in Germanic languages. The initiation point, however, shifts to the following syllable if the distance between the stressed syllable and the end of the word is increased.