1. The stimulating efficiency of hydrochloric, sulfuric, and nitric acids has been measured in the sunfish Eupomotis gibbosus, by a method which reduces experimental errors to a minimum.
2. The results show that stimulation by these acids is primarily dependent upon the (H+) produced in the animal's aquatic environment, and that the reaction time is a logarithmic function of the (H+) within the range tested expressed by the equation: (RT–5) = –4.3 log (H+ x 104) + 9.118.
3. Any effect of the chloride, sulfate, and nitrate ions must itself be measured by the (H+).
4. Variation in the reaction time is also a logarithmic function of the (H+), and the percentage variation is independent of the (H+) over the range tested.
5. Freshly collected fish show a lower threshold for stimulation as determined by the (H+) than do fish adapted to laboratory conditions, but relatively the reaction times of the two groups are the same.