Both the cardiac action potential duration (APD) (0.6 - 1 s) and resting heart rate (30 - 40 bpm) in the horse are significantly different from humans and smaller mammals including rabbit. This would be anticipated to have consequences for excitation-contraction (EC) coupling and require adaptation of the individual processes involved. The sarcoplasmic reticulum (SR) is one of the main components involved in EC coupling. This study examines and compares the activity of this organelle in the horse with that of the rabbit. In particular, the study focuses on SR Ca²⁺ release via the Ca²⁺ release channel/ryanodine receptor (RyR2) and Ca²⁺ uptake via the sarco-endo plasmic Ca²⁺ ATPase pump (SERCA). Isolated cardiomyocytes from both horse and rabbit hearts were permeabilised, bathed in a mock intracellular solution and exposed to a specified [Ca²⁺]. Rabbit cardiomyocytes exposed to 260 nM [Ca²⁺] produced spontaneous Ca²⁺ release and propagated Ca²⁺ waves. Horse cells failed to produce Ca²⁺ waves; instead only local release in the form of Ca²⁺ sparks was evident. However at 550 nM [Ca²⁺], Ca²⁺ waves were produced in both species. Ca²⁺ waves were 4 - fold less frequent yet ~1.5 times greater in amplitude in horse compared to rabbit. Ca²⁺ wave velocity was comparable between species. The reason for this disparity in Ca²⁺ wave characteristics is unknown. Separate measurements of oxalate-supported Ca²⁺ uptake into the SR suggest that both horse and rabbit cardiomyocytes have comparable levels SERCA activity. The possible reasons for the observed differences in SR Ca²⁺ release between the horse and rabbit are discussed.