In situ freezing of the rabbit and rat brain.

We have measured the effectiveness of different standard cryogenic liquids (liquid nitrogen, chilled isopentane, chilled hexane, and chilled ethanol) and applied a mathematical model for calculating the temperature changes at different depths in rabbit and rat heads cooled in liquid nitrogen. We have also evaluated the extent to which the freezing time is shortened when the skin is removed prior to immersion. We conclude that: (i) of the coolants used, liquid nitrogen gives the most rapid freezing. (ii) dry ice-chilled ethanol is as effective as dry ice-chilled hexane. Liquid nitrogen and ethanol are safer for the environment than isopentane and hexane and should satisfy most needs. (iii) By removing the skin prior to immersion, the freezing time can be reduced by about 20 s for the rabbit cortex and rat hypothalamus, 30 s for the rat cortex, and 40 s for the rabbit hypothalamus.