Technical Abstract: Doppler ultrasonography has potential use in studying the effects of ergot alkaloids on vascular circulation in cattle. Accurate and precise measurement of artery lumen area is critical in quantifying vasoconstriction responses to ergot alkaloids and other toxins. Lumen area can be determined by tracing either the inner boundary of the layer of elastin connective tissue in the artery wall (i.e. intima) or the outer boundary of the Doppler flow signal. An experiment was conducted to compare variation between the two tracing methods in measuring cross-sectional lumen area of the caudal artery in five beef heifers on three different dates. Another experiment compared the two tracing methods in detecting changes in artery lumen area after five heifers were switched from a diet containing non-endophyte infected tall fescue (0 µg ergovaline/g of dry matter in diet) to a diet containing endophyte-infected tall fescue (0.85 µg/g of dry matter in diet). Lumen area determined by tracing the Doppler flow signal in experiment 1 was 25% lower than that determined by tracing the connective tissue, but there was no difference (P > 0.10) in the variation of measures between the two methods. Declines in lumen area were detected at the same level of significance (P < 0.05) for both tracing methods after cattle in experiment 2 were switched from non-endophyte infected to endophyte-infected tall fescue diets; however, variation in lumen areas differed (P < 0.05) between non-infected and infected diets for both tracing methods. Results indicated that lumen area of the caudal artery can be measured to the same degree of precision by tracing either the connective tissue in the artery wall or the outer boundary of the Doppler flow signal.