1. The initial heart rate response to lying down was analysed in 18 younger (23-36 years) and 10 older (48-67 years) normal subjects, and consisted of an immediate shortening of the R-R interval reaching a maximum around the third or fourth beat after lying, followed by a lengthening beyond the resting value to reach a steady level around beats 25-30. In six diabetic patients with autonomic neuropathy, no cardiac acceleration occurred and the deceleration was markedly diminished. 2. In eight young normal subjects the pattern of response was altered by atropine, which abolished the initial shortening of the R-R interval over the first 10 beats. Thereafter slow but steady lengthening of the R-R interval occurred. With additional propranolol the later part of the response was further attenuated. Propranolol alone did not affect the normal pattern of response. 3. Six young normal subjects performed short periods of muscular exercise, lying, sitting and standing, and the heart rate patterns were compared with that after lying down. After both manoeuvres R-R interval shortened and then lengthened back to the resting level within 10-15 beats. Thereafter it remained steady after muscular exercise, but continued to lengthen after lying down. 4. In four young normal subjects, no initial R-R interval shortening occurred during fast or slow "passive' tilting from the 80 degrees head-up position to horizontal, whereas shortening was seen both with fast and slow "active' lying down. 5. It is concluded that the immediate part of the heart rate response to lying down (during the first 10 beats) is under vagal control and the later part predominantly under sympathetic control. The first part of the response is probably due to a "muscle-heart' reflex which occurs during the change in posture.