A variety of methods were utilized to evaluate muscle tonus in newborn infants. Healthy newborn infants exhibit a typical posture in the supine position, and a pattern of resistance to stretch in antagonistic muscle groups at a joint. When the biceps brachii was utilized as a test muscle, an inconstant amount of pull was required for full extension. Some degree of tonus, as measured by the amount of pull in grams required for full extension, was present in all infants. The amount of pull required varied as much as 20 times within a single extremity in the same infant when measured at successive hourly intervals. Variability was noted from infant to infant and in the same infant during various activities and in the same infant maintained in varying head positions. Variability of muscle tonus within wide limits (as reflected by the range of resistance to passive stretching of the biceps brachii) appears to be a characteristic of the healthy, newly born infant.