Free ammonia was determined in diaper squeezings from 26 infants with "ammoniacal dermatitis" and in 82 controls. No significant difference was found (402 ppm in diaper dermatitis compared to 465 in controls). The groups did not differ with regard to the incidence of organisms capable of splitting ammonia from urea. Experimental application of highly ammoniacal urine on intact infant and adult skin failed to provoke a dermatitis. Erythema could be induced only when ammoniacal urine was applied occlusively to scarified skin. These findings do not support the notion that ammonia is a primary factor in common diaper rash, but do not exclude a possible role for further irritation in an already existent condition.