In anaesthetized kittens the mean arterial pressure and the pulse pressure double during the first 6 weeks of life; the heart rates recorded were the same as in the adult cat and did not change during this period. The vasomotor sympathetic system was active within 4 days of birth: there was both tonic activity, as demonstrated by a rise in arterial pressure after carotid occlusion, and a rise in pressure during asphyxia; the carotid sinus-cardiac centre mechanism responded to a rise in arterial pressure at birth. All these responses increased with age but had not reached adult activity by the 6th week of life. The carotid sinus-cardiac centre mechanism did not respond to a fall in blood pressure until 4 weeks of age. The infant heart was apparently more sensitive to the direct action of acetylcholine and the peripheral blood vessels relatively less sensitive than in the adult.THE responses both to asphyxia and to the intravenous administration of drugs have been used to examine the activity of the cardiovascular reflexes shortly after birth. The neonatal period has been studied in the kitten and puppy [Clark, 1934], the pig [Adams, Hirvonen, Lind and Peltonen, 1958] and the human infant [James and Rowe, 1957;Adams, Lind and Rauramo, 1958]: observations have also been made in the growing rabbit [Bauer, 1938;Dawes, Handler and Mott, 1957]. The present paper gives an account of a similar investigation into some of the cardiovascular responses of kittens during the first 6 weeks of life when the central arterial pressure is found to increase from 50/40 mm. Hg, at birth, to 120/90 mm. Hg at 6 weeks of age; the cardiovascular responses to positive intrathoracic pressure have been studied; graded doses of the autonomic transmitter drugs have also been used in an attempt to alter the balance between the direct action on the myocardium and peripheral vessels and the regulatory mechanisms involved. The responses are compared with similar observations made on fully grown animals under comparable experimental conditions and discussed in relation to the change in blood pressure during growth. METHODSNineteen kittens, 96-305 g. in weight, aged from 1 to 46 days and eight adult cats, 2-7-4-5 kg. in weight, were used in the experiments. A few observations were also made on five puppies aged 5-110 days, weighing 475 g.-3-3 kg.Aneesthesia was induced with sodium pentobarbitone, 20-30 mg./kg. injected into the saphenous vein exposed after the subcutaneous injection of procaine. The arterial pressure was measured in a carotid or a femoral artery cannulated with a saline-filled polythene tube connected to a capacitance-or inductance-type manometer; recordings were taken with a Sanborn or a New Electronic Products recorder