The experiments presented in this thesis were designed to improve our understanding of subacute ruminal (SARA), rumen function, and their role in milk fat depression. Therefore, the main objective was to develop an animal model to induce a sustainable pH depression. The animal model was utilized to study the association between ruminal pH and ruminal temperature and investigated the effect of ruminal pH on milk fat depression in the presence of polyunsaturated fatty acids. The first experiment was designed to develop a system for the continuous recording of ruminal pH. The system allowed the mobility of animals during recording and was comprised of a heavy-duty pH electrode that was immersed in the rumen and connected to portable pH recorder for data acquisition. Ruminal pH was recorded every minute for 24 hours a day. The second experiment developed the mathematical methodology to predict biological values from ruminal pH measurements. This approach permitted the analysis of ruminal pH measurements from different studies. Continuous ruminal pH measurements from 6 previous SARA studies conducted in our laboratory were analyzed using this approach. This approach improved our understanding of SARA and provided an index to compare the severity of ruminal pH depression across studies. Ruminal pH minima had a high correlation (R2 = 0.77) with ruminal temperature. This indicated that measuring ruminal temperature has the potential to diagnose SARA. The final experiment showed that cows with ruminal pH depression had a greater extent of milk fat depression than cows without ruminal pH depression when soybean oil was infused ruminally. This depression in milk fat was accompanied with a change in the milk fatty acid profile and an increase in the milk concentrations of biohydrogenation intermediates known to depress de novo synthesis of milk fat in the mammary gland.