Penetrability of a single western red cedar (Thuja plicata Donn) stem was investigated using three types of preservatives, namely, coal-tar creosote, 5% penta-chlorophenol solution in oil and 2% Wolman salt (type A) solution in water. The retentions of all three preservatives in the sapwood zone exceeded those in the heartwood. The inner, darker heartwood was superior in penetrability, retaining on the average two to three times more preservatives than did the outer, straw-coloured heartwood. A multiple linear regression technique was used to evaluate the importance of independent variables as they affected the penetrability of the western red cedar heartwood. The following independent variables were considered: age, distance from the pith, growth rate, percentage of latewood, specific gravity, colour, hot-water solubility, and effective one per cent caustic soda solubility (hot-water solubility subtracted from 1% NaOH solubility) of the samples. The colour variable for creosote and pentachlorophenol, and the age variable for Wolman salt were the most important variables. Colour was measured using a Model "F" Agtron electronic colour instrument which expressed the colour numerically on a continuous scale. No thujaplicins - natural preservatives - were found in the pith region of the heartwood, nor in the sap-wood. Around the 50-year age mark thujaplicins began to appear, and their content increased towards the bark, reaching a maximum of 0.835 per cent in the zone next to the sapwood. A fungal infection, believed to be caused by one of the Fungi Imperfect of the white rot type, was found spreading from the pith region outward in the heartwood. In the region where thujaplicin concentration reached 0.361 to 0.549 per cent the fungal infection was not found. / Forestry, Faculty of / Graduate