1. Isolated right and left atria from rats of ages ranging from newborn to adult were used to measure chronotropic and inotropic responses to noradrenaline, isoprenaline, tyramine, and electrical stimulation of intramural nerves. 2. Right atria from newborn animals showed increases in rate with noradrenaline, isoprenaline, and tyramine which did not differ significantly from those of atria from adults. The ED50 values for the chronotropic actions of noradrenaline and isoprenaline were not significantly different at any age from the values in adult preparations. 3. Paced left atria from newborn rats showed well developed positive inotropic responses to noradrenaline and isoprenaline. Newborn left atria (and those from 1 and 2 week old animals) were supersensitive to noradrenaline but not to isoprenaline. 4. Left atria from newborn animals showed very small inotropic responses to both tyramine and field stimulation of intramural nerves. These responses developed progressively with age over the first three weeks of life. The results are discussed with respect to the development of cardiac beta-adrenoceptors and of cardiac sympathetic innervation.