1. To determine the specificity of the goldfish (Carassius auratus) olfactory system to the reproductive pheromone 17 α,20 β-dihydroxy-4-pregnen-3-one (17 α,20 βP), and to determine if related sex steroids might also function as pheromones, electro-olfactogram (EOG) responses were recorded from mature male goldfish. 2. Of the 24 steroids tested, 17 α,20 βP was the most stimulatory. It had a detection threshold of 10-12M and at a concentration of 10-8M elicited an EOG response 3 times that elicited by 10-5M L-serine 3. 17 α,20 β,21-triol-4-pregnen-3-one, a metabolite of 17 α,20 βP, was the only other highly stimulatory steroid. Its threshold was 10-11M. 4. In cross-adaptation experiments EOG responses to all 21-carbon steroids were inhibited during adaptation to 17 α,20 βP: responses to this pheromone are transduced by a single receptor/transduction mechanism. 5. To verify the biological relevance of EOG recording whole animal responsiveness was determined by measuring blood gonadotropin. When goldfish were placed into homogeneous steroid solutions endocrine responsiveness strongly correlated with EOG recording. However, when steroids were added to aquaria containing fish, responses were less specific indicating that transient wisps of steroids trigger endocrine responses. 6. Although the extreme sensitivity and specificity of the goldfish olfactory system to 17 α,20 βP gives it the potential to serve as a highly specific cue, realization of this potential is probably determined by the dynamics of pheromone exposure.