A comparison of the results of Experiments 1 and 2 vividly demonstrates the vulnerability of ram spermatozoa to the stress of freeze-thawing. When ram spermatozoa were subjected to freeze-thaw stress, there was more variation among treatments reflected in maintenance of both intact acrosomes and motile life (Experiment 2, Table 9) than when unfrozen sperm were studied (Experiment 1, Table 5). The influence of glycerol and Tris are particularly noteworthy. Though rate of thaw is not part of the surrounding media, it does control the amount of time the cells are subjected to an even more hostile environment (high salt concentrations) encountered near the melting point of ice. Therefore, the benefit of higher thaw temperatures and resulting faster thaw rates was undoubtedly due to minimizing exposure time of spermatozoa to this adverse condition.