The studies reported here were undertaken to investigate the characteristics of the degradation of two species of fresh forages in ruminants, i.e. lucerne (a legume) and ryegrass (a grass). A review of the literature was done to determine current understanding of the degradation of forage protein by plant and microbial proteases in the rumen. Knowledge of degradability of fresh forages was found to be limited and so three studies were undertaken using forages labelled with 15N so that the end-products of forage protein degradation could be identified and their kinetics determined. The 15N-labelled forage was readily produced by growing the plants in pots watered with a solution of 15NH415NO3. Rumen degradability of fresh and frozen-thawed lucerne and ryegrass was evaluated using in vitro, in situ and in vivo techniques. Therefore, 15N-labelled plant materials can be used to increase the power of in vitro and in situ studies. When used with suitable modelling techniques, in vivo experiments in which animals ingest the forage provide the only means of generating reliable information about the kinetics of ruminal digestion of freshly harvested forages in the rumen.