The period of free flight and weightlessness was characterized by pronounced fluctuations of heart activity in the postacceleration phase. Thereafter, the heart rate of Baker remained relatively constant, whereas the cardiac activity of Gordo fluctuated markedly and decreased slowly almost to the end of his flight. Slight changes, which were transient and not pathological in nature, were also noted in the electrocardiogram. Gordo’s respiration was very shallow during maximum launch acceleration, when Baker’s reached its highest value, only to be approximated again during reentry when forces of about 35 g were encountered. Able’s cardiac and respiratory rates indicated that, after an initial startle reaction, the heart rate dropped transiently and then increased steeply, reaching a maximum of 259 during the 10-second interval at peak acceleration. Respiration increased only slightly throughout the launching phase. There was a period of tachycardia during postacceleration weightlessness, after which the heart rate declined steadily and was disturbed only by several startling missile events. At the end of the subgravity phase, Able’s cardiac rate was slightly below normal. Although the periods of high g force and free flight were short, the extremes were considerable, and the changes from one state to the next were rapid. In spite of this, the cardiovascular, hemodynamic, and electrocardiographic phenomena were remarkably well maintained. Apparently the animals were not in serious plight at any time. That psychological factors entered into the observed phenomena is clearly evident from the increase in cardiac rate associated with the noise of the engine prior to liftoff and also from the cinematographic record of facial expressions. Nevertheless, the integrated responses indicated that the animals’ physiological states remained sufficiently normal to insure a safe flight. LITTLE JOE FLIGHTS The first step in an attempt at animal verification of the adequacy of the Mercury flight program was the development of two tests by NASA in collaboration with the U.S. Air Force School of Aviation Medicine in which there would be a biomedical evaluation of the accelerations experienced during the abort of a Mercury flight at and shortly after liftoff. These flights were launched at the NASA Wallops Station with a Little Joe solid-fuel launch vehicle. Two Little Joe launches were made with activation of the escape rockets during the boost phase to secure maximum acceleration; only a brief period of weightlessness was attained. The first launch was on December 4, 1959, and the other on January 21, 1960. A 36 by 18-inch sealed, 125-pound, cylindrical capsule containing the subject, an 8-pound _Macaca mulatta_, the necessary life-support system, and associated instrumentation was flown in a "boilerplate" model of the Mercury spacecraft. The rhesus monkeys were named "Sam" and "Miss Sam."