In situ incubation of feeds in the rumen serves as a basic procedure in many feed evaluation systems. The formulation of hypotheses concerning underlying biological concepts of ruminal degradability has led to development of different mathematical models describing the resultant time course disappearance curves of feed fractions. The use of models allows comparison of parameter estimates (or combinations thereof), which ideally reflect these biological concepts, between feeds or feeding systems. Among different factors affecting the experimental measurements of in situ ruminal disappearance of feeds, less attention has been paid to choice of mathematical model to fit the curves and the goodness-of-fit of the model. The objective of the present work, therefore, was to evaluate the ability of a zero-order mathematical model to describe in situ disappearance curves obtained with whole soybean, and to give adequate estimates of the parameters needed to determine extent of ruminal degradation.