Wood materials are often used for outdoor constructions. It is expected that with growing demand for natural and renewable materials their usage will even increase in the future. In order to protect the wooden outdoor constructions biocides (wood preservatives) are applied, and if they are released to the environment with run-off waters, this could be considered as a source of environmental pollution with highly toxic substances. Due to the high toxicity, some of the historic wooden preservatives are banned or heavily restricted e.g. creosote, pentachlorophenol. There are several possibilities to reduce the environmental pollution coming from outdoor wooden constructions, e.g., by using wood preservatives with less toxic chemicals as components where possible, by reducing contact with water, or by achieving stronger binding of the toxic substances to the wooden material. The precondition for all these actions is proper information about the presence and concentrations of toxic substances in the composition of wood preservatives. The aim of the present research was to detect the extent of releases of toxic substances from wood materials treated with wood preservatives and to explore efficiency of paint in reduction of leaching of toxic substances. Leachates obtained from painted and unpainted wood materials treated with three different wood preservatives were tested by Daphnia magna ecotoxicity test (LVS EN ISO 6341:1996). All three of the researched wood preservatives were water based and targeted to protect wooden constructions in outdoor conditions. The information about the products applied was crosschecked and verified with various information sources including further testing. Ecotoxicity test results showed that all tested leachates of wood material treated with wood preservative characterizes with high toxicity to D.magna. Treatment of the preserved wood with paint reduces toxicity of leachates justifying painting as efficient measure to reduce leaching of toxic substances. This study was performed in the co-operation among ESF projects 2009/0226/1DP/1.1.1.2.0/09/APIA/VIAA/080 and 2009/0144/1DP/1.1.2.1.2/09/APIA/VIAA/005.