Healthy hearts respond to enhancement of pacing rate by an increase of contractility exhibiting positive force–frequency relations (FFR). In failing hearts rhythm acceleration leads to the progressive decline in force that reflects negative FFR. Our goal is to estimate the effect of [Ca²⁺]_o on the force transient dynamics and resulting FFR in perfused rat hearts. We study in details the transient force responses of Langendorff perfused hearts of rats to step changes in pacing period from 200 to 100 ms and back with a step of 20 ms. We find that the transient responses for both directions are biphasic and can last for more than 3–5 min, indicating that one must be careful in the interpretation of traditional FFR measurements. Also, we study the effects of different [Ca²⁺]_o in Tyrode and Krebs-Henseleit solutions on FFR and show that FFR in rat heart can be changed from positive at 1 mM of [Ca²⁺]_o in Tyrode solution to negative at 4 mM of [Ca²⁺]_o. Furthermore, we find that perfusion with lower [Ca²⁺]_o will give bigger at fast rates and longer lasting transient responses. On the other hand, transients are smaller and less sensitive to pacing period at higher [Ca²⁺]_o. Force–frequency relationships can be analyzed not only upon steady state conditions; the structure of force transient after step changes in pacing period is dependent on extracellular calcium. Our data should be useful in the construction of detailed model of force generation in a cardiac tissue; they can be used to test the validity of models.