Previous reports suggested the existence of direct somatic motor control over heart rate (f(H)) responses during diving in some marine mammals, as the result of a cognitive and/or learning process rather than being a reflexive response. This would be beneficial for O-2 storage management, but would also allow ventilation-perfusion matching for selective gas exchange, where O-2 and CO2 can be exchanged with minimal exchange of N-2. Such a mechanism explains how air breathing marine vertebrates avoid diving related gas bubble formation during repeated dives, and how stress could interrupt this mechanism and cause excessive N-2 exchange. To investigate the conditioned response, we measured the f(H)-response before and during static breath-holds in three bottlenose dolphins (Tursiops truncatus) when shown a visual symbol to perform either a long (LONG) or short (SHORT) breath-hold, or during a spontaneous breath-hold without a symbol (NS). The average f(H) (if(Hstart)), and the rate of change in f(H) (dif(H)/dt) during the first 20 s of the breath-hold differed between breath-hold types. In addition, the minimum instantaneous f(H) (if(Hmin)), and the average instantaneous f(H) during the last 10 s (if(Hend)) also differed between breath-hold types. The dif(H)/dt was greater, and the if(Hstart), if(Hmin), and if(Hend) were lower during a LONG as compared with either a SHORT, or an NS breath-hold (P < 0.05). Even though the NS breath-hold dives were longer in duration as compared with SHORT breath-hold dives, the dif(H)/dt was greater and the if(Hstart), if(Hmin), and if(Hend) were lower during the latter (P < 0.05). In addition, when the dolphin determined the breath-hold duration (NS), the f(H) was more variable within and between individuals and trials, suggesting a conditioned capacity to adjust the f(H)-response. These results suggest that dolphins have the capacity to selectively alter the f(H)-response during diving and provide evidence for significant cardiovascular plasticity in dolphins.