The aim of this study was to determine the vasorelaxant activity of a natural diterpene extracted from Croton zambesicus, ent-18-hydroxy-trachyloban-3-one (DT6), and a synthetic diterpene of similar structure, ent-trachyloban-14,15-dione (DT10) in rat aorta. DT6 and DT10 inhibited aorta contraction in a concentration-dependent manner. Both were more potent inhibitors of KCl-evoked contraction than noradrenaline-evoked contraction. Nitric oxide (NO) synthase inhibition did not significantly affect DT6 effect whereas it significantly decreased DT10 inhibitory potency. In fura-2 loaded aorta rings, DT10 simultaneously inhibited KCl-evoked contraction and cytosolic calcium increase in a concentration-dependent manner. Furthermore, DT10 significantly inhibited calcium channel current recorded by the patch-clamp technique in human neuroblastoma cells SH-SY5Y. However, despite potentiation of 8-bromo-cGMP-response, DT6 and DT10 as verapamil depressed acetylcholine-evoked relaxation, DT6 being the most potent, while only DT6 and DT10 depressed SNAP-evoked relaxation. In conclusion, these data suggest that vasorelaxant activity of diterpenes (DT) is associated with the blockade of L-type voltage-operated calcium channels. Inhibition of NO-dependent relaxation by DT could be related to a decrease in NO availability.