Tversky & Kahneman's Prospect Theory (1981) posits that evaluations of decision problems predictably shift preference when a problem is framed in different ways. This research advances understanding about the effects of message framing on gambling behaviour, an area not yet developed theoretically. Effectiveness of gambling messages (warnings, endorsements) was analyzed by a series of ANOVAs defined by factors including: valence (positive, negative), end-state (positive, negative), and gambler subtype (non-problem, problem gambler). Results (n=228) for warning messages revealed a differential effect of message framing; non-problem gamblers were most persuaded (reduced future gambling intentions) by 'non-gain' messages, and problem gamblers by positive end-states messages - these results attributed to differences in the perceived risk associated with gambling. Endorsement messages were most persuasive (increasing gambling intentions) when positively-valenced for both gambler subtypes. Issue involvement (increasing with gambling severity) amplified message framing effects. Implications for public policy messages to moderate problem gambling are discussed.