The determination of a rate constant for the outflow of chromium-mordanted protein particles from the rumen of lactating dairy cattle. Lactating Friesian/Holstein cows were used for the determination of the outflow rate of protein particles from the rumen. Fishmeal was made non-degradable after treatment with sodium dichromate and was handmixed into the rumen. A total faecal collection was done over 6 days to study the excretion pattern of chromium in the faeces. Faecal marker excretion curves for rate of passage were analysed using the Grovum and Williams model. The outflow rate of protein ranged from 0,022/h to 0,064/h and there was a highly significant linear effect of feeding level on outflow rate (P < 0,01 and R² = 0,97). The following conclusion was made: the level of feed intake is one of the most important factors affecting the outflow rate, if not the dominant factor, and from the literature it is clear that different outflow rates have to be calculated for different types of diets at varying levels of intake and this must be accommodated in feeding standards.