Balancing the rate of nitrogen and energy yielding substrates to rumen micro-organisms has been proposed in order to maximise the capture of rumen degradable nitrogen and to optimise microbial growth rate and efficiency (Johnson 1976). Current systems for diet formulation of ruminants are based upon the extent of fermentation of organic matter or carbohydrate fractions and the supply of rumen degradable and undegradable protein (ARC 1984, Webster, Dewhurst and Waters 1988). Little attention has been paid to the effect of the rate of supply of energy and nitrogen to rumen microorganisms in-vivo. The objectives of the current experiments were to formulate diets that had similar daily degradation of nitrogen and energy yielding components but varied in the rate of energy and nitrogen release based upon in-situ degradation characteristics of the raw ingredients, and to monitor the effect of these diets on rumen fermentation and microbial growth.