In the absence of other factors known to influence sectioning properties, high environmental relative humidity is shown to yield poorly embedded tissue. Humidity-related effects are avoided if the following embedding precedure is used; impregnate tissues using the following solutions 1) 70% alcohol - 5 minutes, 2) 95% alcohol - 2 x 15 minutes, 3) absolute alcohol - 3 x 20 minutes, 4) acetone - 2 x 15 minutes, 5) 1:1 mixture of acetone-epoxy resin (DDSA, 63.4 g; Araldite 502, 5.6 g; Epon 812, 39.4 g; DMP-30, 2.6 g) - 1 hour, 6) acetone-epoxy resin 1:3 - 1 hour, 7) epoxy resin - 1 hour; complete the preparation of blocks as follows 8) when tissues have been oriented in epoxy resin in flat embedding molds, place molds in one evacuated vacuum desiccator 10 cm above a 2 cm layer of Drierite for 24 hours at room temperature, 9) raise temperature to 60 C and maintain for 3 days to cure resin.