Three vasoactive drugs--glucagon, acetylcholine and dopamine were used in calves with an implanted TAH to study their mere vascular effects. We noticed that glucagon, besides being a potent vasodilator, affected the vascular system gradually rather than instantly. Also, it allowed for the gradual recovery of the system, thus avoiding sudden fluctuations in systemic and pulmonary blood pressures. Glucagon was found to have a unique capability of decreasing vascular resistance with only a slight drop in systolic pressure. Acetylcholine action was rather instant, exhibiting a sudden fall in both systolic and diastolic blood pressures. In animals with a TAH, dopamine seemed to work differently from that in subjects with an intact natural heart. While many investigators failed to see any change in pulmonary vascular resistance due to dopamine injection in subjects with an intact natural heart, we noticed about 20% increase in pulmonary vascular resistance (with a dose of 10 muGm/Kg). However, at low dosages of dopamine, we did not notice any appreciable change in pulmonary vascular resistance, which suggests that its action is dose dependent. We tend to believe that glucagon offers promising results in controlling the pulmonary hypertension in calves with a TAH. It is, also, our opinion that further investigation on the vascular effects of dopamine would reveal more pertinent and interesting results and solve the existing discrepancies.