PH20 (IPT (International) Ltd, Newark) is a water-insoluble polymer gel of acrylic acid which has been shown to improve the digestion of silage in the rumen, leading to improved live-weight gain and feed conversion efficiency in beef cattle (Garnsworthy, 1987) and, on a large number of commercial dairy farms, has led to savings in concentrate usage. Increases in milk yield have been found on only a minor number of farms and it appears that with cows in mid to late lactation unless the concentrate allocation is reduced, feeding PH20 will result in greater deposition of body reserves. The objective of the trial reported here was to study the response by cows in late lactation to PH20 under grazing conditions.