SUMMARY: Female rats consistently preferred the odour of male rat preputial gland compared with that of foot pads, submaxillary-sublingual glands, coagulating glands, liver, fat or muscle. Both saline homogenates and ether extracts were effective. Female rats did not respond to the odour of female preputial extract and they preferred the odour of normal male preputial extract to that from castrated rats. The pheromone was not associated with the fatty acids of the preputial extract. The fractionation of the volatile components of preputial extracts by gas chromatography revealed that most of the biological activity resided in a specific fraction.