Four West African dwarf wetl1ers fitted with permanent rum cannulae were used to study the effect of supplementing hay with concentrates of varying protein contents on the concentration of total nitrogen (TN.), total protein nitrogen (TPN) non-protein nitrogen (NPN) ammonia nitrogen (AN) residual nitrogen and free amino nitrogen in the rumen, and of urea nitrogen in blood plasma. The amino acid composition of ruminal micro-organisms was also determined. Total ruminal ranged from 29·9mg/100ml to 124·9mg/ 100ml and mean differences were highly significant (P<0·01). The ranges of the concentrations of other metabolites were: TPN 21·7 to 79·1; NPN, 8·2 to 45·8; AN, 1·2 to 4·7; residual N, 6·7 to 41·3mg/100ml and free amino, 1·6 to 6·3 ?mole /ml. Plasma urea N concentration were highest with the hay diet (5·3mg/100ml). The level of AN ?-amino and plasma urea N were depressed when hay was supplemented with low protein concentrates. The amino acid histidine and methionine were present in low concentrations and 15 to 20 per cent of microbial protein was present as glutamic acid. Ruminal ammonia concentrations were probably too low for optimum microbial activity. It is considered that when available carbohydrates are used to supplement low or medium quality forage, urea or other non-protein could also be included to give optimum ruminal conditions for microbial growth.