Kinetics of particle and fluid ruminal passage were studied in steers grazing signal grass (Brachiaria decumbens Stapf.). Analyses of variance were performed according to a completely randomized block design including treatment (dry and rainy seasons) and block effects (animals) in the model. The first analysis was done based on the effects of the particle phase markers (chromium, ytterbium and europium) while the second analysis relied on the mathematical model effect that make up the sub-plots. For fluid kinetics, treatments (four replicates) were assigned to a completely randomized block design in a split-plot arrangement with the experimental period in the main plot and the mathematical models in the sub-plot. Models were compared using residuals. The multicompartimental model was the best fit to the observed data and chromium-mordant the best particulate phase marker with passage rates estimates of 0.0283 and 0.0521 h^-1 in the dry and rainy seasons, respectively. The best fit for passage of fluids was the double compartment exponential model that estimated fluid passage rate of 0.1183, 0.0947 and 0.1342 h^-1 for July, September and January, respectively. For ruminal degradation kinetics, treatments (rainy and dry seasons) were assigned to a completely randomized block design (four replicates) in a split-plot arrangement with the experimental period in the main plot and the parameter estimates obtained by fitting different mathematical models in the sub-plot. The comparisons among models were also done by residual analysis. Effective degradability of forage dry matter was greater in the rainy season (327 gkg^-1) than in the dry period (279 gkg^-1). The degradation profile of the forage dry matter was completely explained by the first-order asymptotic growth model. To describe fiber degradation, the best mathematical models fit were the first-order asymptotic growth corrected for discrete lag and its reciprocal. Effective degradability of fiber was 199 and 232 g for dry and rainy seasons, respectively.