The dose-activity relationship of various positive inotropic drugs has been investigated in the spontaneously beating right atria of the rat. The influence of dose regimen and age has been evaluated. There is a significant difference between the maximal response of single and cumulative dose experiments with amrinone (P less than 0.05) and dobutamine (P less than 0.001) in animals aged 2 months and in dopamine (P less than 0.001) in animals aged 4 months. The maximum response is lowered significantly with age for isoprenaline with single (P less than 0.05) and for dopamine and dobutamine with cumulative dosing (P less than 0.001). In the single dose study the response is achieved within 4 min. for isoprenaline, noradrenaline, dobutamine, xamoterol and amrinone, but not until after 14 min. for prenalterol. It is concluded that both age and methods used in the study of pharmacodynamics are important. Thus when comparing effects the use of the same method and tissue from rats of the same age is necessary.