The present study attempts to highlight the principles by which peripheral olfactory information of across- and within-class odorant signals is transformed into bulbar neuron responses. For this purpose, we performed electro-olfactogram cross-adaptation and mixture experiments as well as single unit recording of olfactory bulb neurons using amino acid, bile acid and F-prostaglandin stimulants in brown and rainbow trout. The results show that amino acids, a bile acid and a F-prostaglandin activate independent receptor types. However, within the class of amino acids, different receptor types are only partially independent. Neurons responsive to bile acid and amino acids were segregated to the mid-dorsal and latero-posterior olfactory bulb, respectively. Of the 43 responsive olfactory bulb neurons studied in brown trout, 41 showed specificity for one odorant class. Olfactory bulb neurons gained responsiveness to new amino acids with increasing stimulant concentration. We conclude that different odorant classes activate specific neurons located in different regions of the trout olfactory bulb, and that information distinguishing related amino acids can be represented in a limited number of bulbar neurons with distinct response profiles under the conditions investigated.