Urbanparksareimportantsettingsforphysicalactivity,butfewnaturalexperimentshaveinvestigated the influences ofparkmodifications onactivitypatternsandvisitorprofiles.Weassessedtheimpactof implementing amunicipalpolicyonoff-leashdogsincityparksinCalgary(Alberta,Canada).Systematic observationundertakenin2011and2012withinfourparkscapturedpatternsofuse,activities,and visitors' characteristics.Afterbaselinedatacollection,off-leashareaswerecreatedintwoparksonly.We compared changesinthesociodemographicandactivityprofiles inallparksbetween2011and2012. Visitors with dogs participatedinlessintenseactivityrelativetovisitors without dogs. Inbothmodified parks, theintensityofchildren'sactivitiesdecreased,whiletheintensityofadults’ activities remained stable. Adjustingforvisitorcharacteristics,thelikelihoodofdog-relatedvisits,relativetootheractivities, significantly decreasedinoneofthetwomodified parks(oddsratio0.55, po.05). Accommodatingoff- leash dogsinparkshasthepotentialtomodifyactivitiesundertakeninsideparksaswellastheprofile of visitors, butmaynotincreaseparkvisitsamongdog-walkersintheshortterm.Recreation,park,and urban plannersandpolicy-makersneedtoconsidertheneedsandpreferencesofthebroadercommunity in thedesignandredesignofpublicparks.