The effect of feeding pigs weaned at 24 & 2.6 days, ad libitum, a diet mixed with water to provide four different dry matter concentrations of 149, 179, 224 and 255 g kg-' (diets DM149, DM179, DM224, DM255, respectively) was investigated in a 28 day feeding trial, utilising a total of 96 pigs. The diets were delivered by an automated system, the storage and mixing vessel of which was replenished daily. In addition to the water supplied in the liquid diet water was freely available to the pigs at all times from nipple waterers. Dietary dry matter (DM) concentration had no significant effect on DM intake, weight gain or DM feed conversion ratio of the piglets. Decreasing feed DM had a noticeable effect on total volumetric (DM plus water) intake of the piglets. When feed dry matter was reduced from 255 to 224 g kg-' piglets reduced their voluntary water intake from nipple waterers in order to maintain DM intake and total volumetric intake. At lower DM concentrations, the pigs maintained DM intake by increasing their total volumetric intake. Piglets continued to consume an average of 223 ml water per day from the nipple waterers even at the lowest DM concentration. On DM149 total volumetric intake was 30% of live weight. The effect of dry matter (DM) concentration of feed on the pattern of microbial activity of the liquid feed system was investigated. Liquid diets were colonised rapidly by naturally occurring Lactobacillus spp in the first 5 days. After 5 days the population stabilised and remained constant for the remainder of the trial period. DM concentration of liquid feed had little overall effect on the pattern of microbial activity. The proliferation of Lactobacillus spp resulted in acidification of the liquid feed, (pH range 4-1-4.2) for all four treatments and this was associated with a lowering of coliform numbers. There was a significant increase in the overall effluent output as the DM content of the diet decreased (P < 0.01). It is concluded that weanling piglets will readily accept liquid feed with DM content in the range 255 to 149 g kg-', and that DM content in this range has little effect on post weaning performance. However, in order to maintain DM intake without increasing effluent output, it is recommended that the DM content of liquid diets should not be reduced below 200 g kg-' (equivalent to a 3.5 : 1 water to feed ratio).