The majority of children are not meeting physical activity guidelines (PAG) or screen time guidelines (STG). Parents play a role in changing the health behaviours of their children by engaging in supportive behaviours (e.g., encouraging sports; limiting TV viewing). According to the social issue advertising believability model (SIABM), whether parents support their children in achieving these guidelines may depend on how believable they find guideline promotion advertisements (message believability [MB]). Whether parents find a message believable may depend upon their perceived behavioural control for engaging in supportive behaviours. The purpose of this study was to examine a) MB as a determinant of parents’ intentions to support their children in meeting the PAG and STG, and b) perceived behavioural control as a determinant of whether parents consider PAG and STG advertisements believable. A secondary objective was to examine differences in SIABM variables for PAG versus STG advertisements and for moms versus dads. Data were collected online using Survey Monkey Audience; 500 Canadian parents with at least one child aged 5-11 (75.6% moms) were recruited. A structural equation model testing the SIABM relationships demonstrated good model fit for both advertisements in the whole sample, and moms, and dads separately (CFIs≥.96; RMSEAs≤.06). Contrary to hypothesis MB did not predict intentions, and perceived behavioural control did not predict MB (ps>.05). ANOVAs revealed that parents had more positive evaluations of the PAG advertisement versus the STG advertisement; yet, had stronger intentions to support screen time reduction versus physical activity participation (ps<.05;d=.24). When considering moms and dads separately, moms believed, and had stronger attitudes towards, both advertisements than dads. Moms had greater intentions to support screen time reduction versus physical activity participation. Given that parents believe the guidelines, efforts must be directed to enhance other determinants of parental support behaviours – particularly screen time reduction.