The studies reported in this thesis 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 processes 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 ¹⁵NH₄¹⁵N0₃. Rumen degradability of fresh and frozen-thawed lucerne and ryegrass was evaluated using 'in vitro', 'in situ' and in vivo techniques.