The possibility that the cardiac SR undergoes developmental changes at about the time of birth, and that these changes affect its ability to accumulate Ca2+ and to hydrolyse ATP has been studied. SR-rich microsomal fractions were prepared from heart muscle excised from foetal guinea pigs and rabbits 1 day before their anticipated date of birth, and from 1 day old and adult animals. For control purposes microsomes were also prepared from the relevant maternal stock animals. One day before birth the cardiac microsomes of the foetal but not of the maternal animals exhibited a decreased ability to accumulate Ca2+ by uptake but not by the binding process, and a decreased ability to hydrolyse ATP. This reduction in ATPase activity involved both the Ca2+-dependent and the Ca2+-independent ATPase enzymes. One day after birth the Ca2+-accumulating activity of the neonatal microsomes had increased, that of the rabbit via an increase in Ca2+ uptake and that of the guinea pig by an increase in Ca2+ binding. These changes were accompanied by an increase in the activity of the Ca2+-dependent ATPase. The results are interpreted to mean that the cardiac SR changes at about the time of birth, and that although the pattern of these changes may be species specific they result in an increase in the Ca2+-accumulating activity of the SR.