The ability of the turtle olfactory system to discriminate between various odorants that increase levels of adenosine 3,5-cyclic monophosphate (cAMP) and mositol trisphosphate (IP ₃) in the olfactory bulb was examined by the cross-adaptation technique and analyzed by multidimensional scaling. The mean values of the degree of discrimination among the IP ₃-increasing odorants were higher than those among the cAMP-increasing odorants, and were similar to those between cAMP- and IP ₃-increasing odorants, suggesting that the features of the receptors of cAMP-increasing odorants are different from those which respond to IP ₃-increasing odorants. Analysis by multidimensional scaling suggested that differences in second messenger pathways are not related to detecting odor quality in the turtle olfactory system.