Variations in left ventricular stroke volume (SV) is an important cardiovascular variable seldom investigated. This thesis in cardiovascular physiology studied the variations in SV in healthy, young volunteers. The focus was the role of SV variations in short-term regulation of the circulation. The circulatory changes that occur during mild exercise were investigated as were the spontaneous cardiovascular oscillations. The main protocols were experimental studies on humans, and a mathematical simulation model tested a hypothesis of cardiovascular adaptations at the onset of exercise. SV was measured by ultrasound Doppler allowing a high time resolution, and every cardiovascular variable was recorded beat by beat. Thus, short term regulations have been investigated by these methods with a time resolution down to one heart beat. During mild supine exercise SV decreases due to an increase in blood pressure, and there is a redistribution of blood flow even when heart rate is below 90 beats per min. In addition, baroreflex resetting and local vasodilation are sufficient to produce the cardiovascular adaptations observed at the onset of exercise. Cardiovascular rhythms are cyclic fluctuations in cardiovascular variables such as heart rate and blood pressure. We studied cardiovascular oscillations related to respiration (related to respiratory sinus arrhythmia) and the ten second rhythm in blood pressure (Mayer waves). We found that respiratory sinus arrhythmia buffer blood pressure oscillations. We also suggest that blood pressure oscillations are replaced by heart rate oscillations in a healthy cardiovascular system. In conclusion, SV is an important variable, and SV is highly sensitive to blood pressure changes. The combination of variations in heart rate and SV reduces fluctuations in cardiac output. This thesis sheds new light on SV variations, and measurements of SV variations may represent an important tool for evaluation of the circulatory system in the future.