This paper presents an experimental investigation and evidence of rate-dependency in the planar mechanical behaviour of semilunar heart valves. Samples of porcine aortic and pulmonary valves were subjected to biaxial deformations across 1000-fold stretch rate, ranging from 0.001 to 1s-1. The experimental campaign encompassed protocols covering (i) tests on samples without preconditioning, (ii) preconditioning immediately followed by tensile tests; and (iii) tensile tests at different rates performed on the same preconditioned specimen. Our results indicate that under all employed loading protocols, heart valve samples exhibit a marked rate-dependency in their deformation behaviour. This rate-dependency is reflected in stress-stretch curves and the calculated ensuing gradients, where samples typically show stiffening with increased rate. These results underpin one conclusive outcome: the in-plane mechanical behaviour of semilunar valves is rate-dependent (0.055 for Cauchy stress levels 50 kPa). This outcome implies that the rate of deformation for characterising the mechanical behaviour of semilunar heart valves may not be chosen arbitrarily low, and models that incorporate rate-effects may be more appropriate for better capturing the mechanical behaviour of heart valves.