In reviewing the blood borne "mediators" to ventilation during exercise, our emphasis is on VCO2 and [H+]. We consider that exercise ventilation is more closely linked to VCO2 than to VO2. Lactic acid has a dual effect on ventilation, as it results in an increased CO2 output as well as increased [H+], an independent ventilatory stimulus. Examples from acid-base disturbances hypoxia and training are used to illustrate the relationship of VE to VCO2 and [H+].