The objective of this study was to investigate the effect of method of delivery of a solution contaning cellulase on the in vitro rumen fermentation (24 h) of three tropical forages. Enzyme was applied to forages either at the time of incubation or 24 h before. Both cellulase treatments increased (P<0.05) acetate, propionate and total VFA production, as well as neutral-detergent fibre degradability (NDFD) with all forages. NDFD increased significantly (P<0.05) with all forages when 24 h pre-treatment of forage with the enzyme was allowed, and this pre-treatment also tended to increase (P<0.05) gas production with two of the three forages. The results indicate that 24 h pre-treatment of forages with cellulase could increase the efficacy of this enzyme, but the incubated forage affects the results.