Background Little is known about how General Practitioners (GPs) decide whether to participate in emergencies. Aim To test whether GPs participation is associated with cause of symptoms, distance to the patient, other patients waiting and out of hours (OOH) clinic characteristics. Design and Setting Online survey to all Norwegian GPs (n = 4701). Method GPs were randomised to vignettes describing a patient with acute shortness of breath and asked if they would participate in a call-out. The vignettes varied with respect to cause of symptoms (trauma versus illness), distance to the patient (15 versus 45 minutes) and other patients waiting at the OOH clinic (crowding versus no crowding). The survey included questions about OOH clinic characteristics. Results Of the 1013 GPs (22%) that responded, 76% reported that they would participate. The proportion was higher in trauma (83% versus 69%, chi square 24.8, p < 0.001), short distances (80% versus 71%, chi square 9.5, p = 0.002) and no crowding (81% versus 70% chi square 14.6, p < 0.001). Participation was associated with availability of a manned response vehicle (adjusted odds ratio [OR] 2.06, 95% confidence interval [CI] 1.25-3.41), and team training at the OOH clinic once a year (OR 1.78, 95% CI 1.12-2.82) or more than once a year (OR 3.78, 95% CI 1.64-8.68). Conclusion GPs were less likely to participate when the incident was not due to trauma, was far away and when other patients were waiting. A manned response vehicle and regular team training were associated with increased participation.