The present investigation examines intra-speaker variability in heritage speakers (HSs) of Spanish by focusing on the potential effects of two variables in their acquisition of mood: lexical frequency and morphological regularity. To do so, this study tested participants' interpretation and use of early-acquired mood alternations conveying either assertive or jussive meanings. Results from a truth-value judgment and a picture-based sentence completion task revealed that HSs' performance was significantly modulated by the lexical frequency of the matrix verb introducing the modal alternation, as well as by the regularity of the embedded verbal form. In particular, frequent matrix verbs and irregular forms yielded higher rates of accuracy across most of the experimental tasks, suggesting that the degree of variability in HSs' outcomes is determined, to a certain extent, by these two factors. These results favor an account of heritage language acquisition and loss focused on the effects of lexical activation on bilinguals' grammatical representations (Perez-Cortes, Putnam and Sánchez, 2019; Putnam and Sánchez, 2013).