Laboratory, digestion and lactation trials investigated the replacement value of the tree legumes Gliricidia sepium and Leucaena leucocephala for a concentrate supplement in kinggrass (Pennisetum purpureum) based diets. In the lactation trial, 15 does in mid-lactation were allocated to three dietary treatments in a 3 × 3 Latin square design with five square replications. In the digestion trial, six intact yearling male goats were assigned to the same treatments in a replicated 3 × 3 Latin square design. Diets were based on kinggrass and supplemented with either a commercial concentrate feed, gliricidia or leucaena. In the control diet, 60% of the total dietary N was supplied by the concentrate. In the legume-supplemented diets, gliricidia or leucaena replaced approximately 50% of the N supplied by the concentrate. Dried banana fruit (Musa×paradisiaca) was used in the legume-supplemented diets to equal the energy content of the control diet. Diets contained approximately 2.2% N and 73% total digestible nutrients. Legumes were greater in total N concentration than kinggrass, with leucaena containing more N than gliricidia (P<0.05). Size and degradation rate of the potentially ruminally degradable N fraction were greatest for gliricidia and least for kinggrass, with values for leucaena being intermediate (P<0.05). In vitro organic matter (OM) digestion was greater (P<0.05) for the legumes than for kinggrass, but the extent of in vitro cell wall digestion was greater (P<0.05) for kinggrass than for the legumes. In vitro OM and cell wall digestion was greatest (P<0.05) for banana. In the digestion trial, apparent dietary OM and cell wall digestibilities were similar (P>0.10) across treatments. Fecal N production was greater (P<0.05) for legume-supplemented diets than the control, leading to lower (P<0.05) N digestibility for legume-supplemented diets. In the lactation trial, OM intake was greatest (P<0.01) for the control diet and least (P<0.01) for the gliricidia-supplemented diet, but milk production characteristics were similar (P>0.10) for all treatments. For iso-energetic diets, up to 50% of the concentrate N may be replaced by the tropical tree legumes gliricidia and leucaena without a reduction in milk production.