1. Noradrenaline has paradoxical concentration‐dependent effects on the Purkinje fibre action potential. At concentrations greater than 500 nM, it shortens the action potential whereas at lower concentrations it prolongs the action potential. 2. We determined the dose‐‐response relation for Isi to noradrenaline in Purkinje fibres and found that a noradrenaline concentration of 126 nM produces a half maximal effect. 3. A maintained component of Isi was shown to have a similar response to noradrenaline. 4. The dose‐dependence of noradrenaline‐induced changes in Ix was determined. The half maximal noradrenaline concentration for this relationship was shown to be 133 nM. 5. The dose‐‐response relationships for Ix and Isi were not altered by variation in extracellular Ca2+. 6. Ix appears to be approximately twice as responsive as Isi to saturating doses of noradrenaline. 7. These data were incorporated in a computer‐generated reconstruction of the effects of noradrenaline on the action potential and are sufficient to account for the paradoxical effects of noradrenaline.