Phonological tone-vowel interactions are cross-linguistically rare. The attested interactions are often problematic because they could be interpreted by mediation of some other feature or prosodic constituent. This paper focuses on tone-ATR interactions in Slovenian that do not suffer from these challenges. We show that the combination of High tone and lax mid vowels is dispreferred in Slovenian. We attribute this preference to a markedness constraint. In the native phonology, the constraint causes mid lax vowels to surface with a Low tone. In the loanword phonology, where High tones are required, the same constraint causes mid vowels to surface tense. This heterogeneity of process and homogeneity of target is a hallmark of markedness constraints.