The first part of the study consisted of 110 rats in 11 groups with ten rats in each. Nine of the groups were fed nutrient solutions of different compositions, antacid and sucralfate through orogastric tube during induction of stress ulcer by restraint and a cold ambient temperature. One group served as a control group and received no feeding and the 11th group was given cimetidine intraperitoneally. The extensiveness of the stress effect was estimated in each group by the number of rats with ulcer as percentage, the mean number of ulcers in each rat, the mean distribution of ulcers of different sizes, the mean total of mucosal damage in each rat and the contribution of ulcers of a different size to the total mucosal damage. The results showed that cimetidine is an effective protector against stress ulcer. Guar gum, Intralipid (fat emulsion), egg protein and 30 per cent glucose are slightly weaker protectors than cimetidine but much stronger than 10 per cent glucose wheat flour and distilled water. Sucralfate increased the susceptibility to stress ulcer. The second part of the study consisted of 86 rats. It showed, that guar gum increased the healing rate of stress ulcers. During a 30 hour treatment period after four hours of stress, the rats fed guar gum (n = 30) showed a lower (p less than 0.001) number of ulcers than the control rats fed normal rat food (n = 26) or immediately after the four hours of stress (n = 30). The mechanisms suggested for ulcer prevention and increased ulcer healing rate found herein may be due to reduced acidity, increased local mucosal supply of energy and mechanical protection.