Sources of non protein nitrogen (NPN) are attractive because of their low cost relative to vegetable proteins. Urea is the most commonly used NPN source and is rapidly hydrolysed to ammonia within the rumen. A very rapid release of ammonia may result in reduced utilisation by ruminal bacteria, increased N excretion, negative impacts on fertility and potentially ammonia toxicity (Richardson et al., 2003). This has lead to the development of slow-release urea based products including isobutylidene diurea, acetylurea, biuret and formaldehyde treated urea. Recently a blended fat and urea source (Optigen®) has been shown to reduce the rate of ammonia release in the rumen, but its effects on microbial metabolism are unclear. The objectives of the current study were to evaluate the effects of a slow release urea source (Optigen®) as a partial replacement for soyabean and rapeseed meal on microbial growth and metabolism in vitro.