For the first time, spatial and temporal distributions of Hg, Cd, Pb, Cu and Zn trace metals in seawater, sediment and biota of the Mljet National Park were investigated. Distributions indicated sources and sinks of these metals in seawater and sediments of the Mljet NP. Concentrations of investigated trace metals in sediments were within the range found earlier in Croatian Adriatic coast. Natural and anthropogenic factors play an important role in determining resultant trace metals concentrations in the area. Careful study followed by suitable interpretation based on geochemical data, were necessary to distinguish natural from anthropogenic sources.