Potentiation of heart muscle stripes of the right ventricle was investigated in normal rats (NT-group) and in rats stressed by swimming (HT-group). The mean fibre diameter runs to 26 micrometer in HT-group and 16 micrometer in NT-group. The difference was significantly high, as well as that of relative heart weights. The potentiation by paired pulse stimulation was more marked in HT-group than in NT-group. The interval-strength-relationships increase with prolongation of the driving interval. Potentiation is greater in loaded rats than in normal rats. A mathematical model for Ca++-storage in sarcoplasmic reticulum is introduced. Decrease of basic-filling of Ca++-store is modelled to HT-group with support of morphological measurements and is discussed as reason for improved properties of potentiation of HT-group.