Signalling with intracellular enzymatic cascades is a complicated process in cardiac myocytes. Periodic, pulsatile changes in intracellular Ca²⁺ concentration have a crucial role in activation of several of these pathways. We present a mathematical model illustrating the control of enzyme kinetics in rat cardiac myocytes. The model development was based on previous work (Bhalla & Iyengar, 1999; Tavi et al. 2003, 2004). The model includes intracellular Ca²⁺ handling as well as interactions between calmodulin (CaM), calcineurin (CaN), protein phosphatase 1 (PP1) and calmodulin kinase II (CaMKII). Simulations using both the experimental calcium transients from isolated ventricular myocytes and the simulated transients indicate that the model consistently predicts time courses of enzyme activations in agreement with other available experimental data. In previous research projects (Tavi et al. 2003, 2004), we have shown this type of modelling to be useful in understanding experimental results related to CaN and CaMKII activation. The new model increases our understanding of the role of PP1 in the interaction between CaN and CaMKII and of the role of localization of CaM. Our model provides novel ways to study the enzyme cascades, e.g. the effect of durations, amplitudes and intervals of the intracellular calcium signals may be varied in the physiological range of parameters. It also makes it possible to go beyond the physiological range of stimuli and reaction parameters, which may be useful in revealing hidden aspects of signalling. Our results show what is the frequency dependence of the Ca²⁺ dependent enzyme activation. The modelling also shows the differences of enzyme signalling in the cytosol vis-à-vis the nucleus.