Background. Building on self-determination theory (Deci & Ryan, 2000), this study examines the mediating role of basic psychological need (BNP) satisfaction and thwarting for school principals between quality of interpersonal relationships (QIR; Senécal et al., 1992) with three referents (superintendant, school personnel, and parents) and their optimal functioning (i.e. vigor [Schaufeli et al., 2006], organizational commitment – affective dimension [Meyer et al., 1993], and subjective job performance [Williams & Anderson, 1991]). Although many empirical studies have confirmed that social environment plays a key role in employees’ personal growth through BPN satisfaction (relatedness, competence, and autonomy), very few studies have simultaneously looked at the competing roles of both BPN satisfaction and BPN thwarting as mediating variables (Deci et al., 2017). Hypotheses. This study contributes to research by providing insights into the differential mediating roles of BPN satisfaction and BPN thwarting in the association between principals’ QIR and their optimal functioning. Sample (size and characteristics). Data were collected from 143 school principals (72.0% women; Mage=47.8; SD=6.6) working in three School Boards in Quebec (Canada). Results. The model was validated with path analysis and the analyzed results display satisfactory fit indices (χ2 = 10.285 [df = 9]; CFI = .997; TLI = .979; SRMR = .027; RMSEA = .032 [CI=.000-.102]). Results suggest that QIR positively predicts satisfaction for the three needs and, negatively predicts their frustration. The model shows four significant mediating paths: 1) QIR/superintendant → autonomy satisfaction → commitment, estimate = .104, p < .006; 2) QIR/school personnel → relatedness satisfaction → vigor, estimate = .087, p < .026; 3) QIR/school personnel → autonomy satisfaction → commitment, estimate = .053, p < .050; 4) QIR/parents → competence satisfaction → job performance, estimate = .114, p < .015. The theoretical and practical implications of this study will be addressed throughout the presentation.