The Function Acquisition Speed Test (FAST) has shown recent evidence as an effective tool for the quantification of stimulus relatedness. The current study assessed the potential of the FAST in measuring the effects of the presentation of positively or negatively valenced messages on relatedness between stimulus relations with regard to safe-sex behavior, namely condom use. Fifty-one participants were assigned to one of three conditions comprised of valenced message interventions regarding the impact of condom use on the enjoyment of sexual behavior (each condition n = 17): a positive-message condition, a negative-message condition, or a no-message control condition. A significant Strength of Relation (SoR) score was found across positive and negative FAST test trials in the positive-message condition only, with no significant differences in SoR scores observed for either the Negative-message or Control conditions. These data suggest that the FAST may have utility as a sensitive behavioral tool for measuring changes in stimulus relations concerning safe-sex behavior on the basis of brief message interventions.