It is important to understand the protein hydrolyzing systems of barley and malt so that improved malting barleys can be developed and enhancements to the malting and brewing processes can be attained. The proteases of barley and malt partially hydrolyze storage proteins during malting and mashing, releasing amino acids, peptides and soluble proteins that ultimately affect many facets of brewing such as the taste, haze, color, mouth feel and foam of beer. During malting (germination), the proteolytic activities begin increasing after the first day of germination and the enzymes that form were totally preserved throughout kilning at temperatures up to 85°C. This study was undertaken to find out when the proteolytic activities were inactivated during mashing. Experimental mashes were prepared using methods that emulated commercial brewery practices. Samples were removed from the mash at appropriate times and their activities were analyzed using both qualitative (electrophoretic) and quantitative (in vitro analysis) methods. The malt protease activities were stable throughout the 38°C protein rest phase, but were quickly inactivated as the mash temperature approached and reached the 72°C conversion step. The rates of inactivation of the proteases were also measured. The results of these studies indicate that, since the proteases are completely stable throughout the protein rest, the soluble protein levels of worts can be varied by adjusting length of this phase of mashing. They cannot be adjusted by altering the conversion time, however, since they are inactivated very early in this process. The amounts of the various malt proteins that are hydrolyzed during mashing probably cannot be changed very much by modifying the mash temperature schedule, since most of the proteases that catalyze these hydrolyses are apparently inactivated similarly as the temperatures are increased.