Female mice were reared in observation incubators from day 1 of life for three weeks. During that time they were continuously exposed to the odors of either cyclooctanone, adult male mouse urine or distilled water. The growth rate was temporarily accelerated for the cyclooctanone-exposed mice. There was no difference in age at sexual maturation of the three groups. Olfactory preference, when adult, was not affected by early odor exposure, but sniffing behaviour was markedly increased in the urine-exposed mice as compared with the other two groups. The olfactory bulbs of the cyclooctanone-reared mice were larger than those of the other two groups. Mitral cells in the olfactory bulbs were examined histologically for abnormalities. All mice had some shrunken, darkly staining mitral cells, but the cyclooctanone-reared mice had twice as many as the other two groups, mainly in the dorsal half of the bulb. The urine-exposed mice also had more darkly staining cells than the control mice particularly in the dorsolateral region of the bulb, but also in the dorsomedial region.