1. The reaction of the sunfish, Eupomotis gibbosus, to different concentrations of hydrochloric acid and of the first six members of the N aliphatic acids has been studied.
2. The stimulating efficiency of hydrochloric acid may best be related to the concentration of hydrogen ions produced by that acid.
3. The stimulating efficiency of the N aliphatic acids may best be correlated with the non-polar nature of a portion of the molecule, but it is necessary to consider the higher potential of the polar group of formic acid to account satisfactorily for its position in the series.
4. When equally effective concentrations of the N aliphatic acids are compared, formic acid is more effective at lower concentrations than at higher.
5. Per cent variation in response appears to be independent of the chemical environment to which the animal responded.