Neurofibrillar changes occur in the brains of goldfish, Carassius auratus L., maintained at 5 degrees C for 177 days or more under a 12-hour photoperiod. In paraffin sections impregnated by the silver method of Holmes, the light microscopic appearance of the neurofibrillary change was evidenced by black rings 1 micrometer to 2 micrometer in outside diameter. A quantitative study showed the mean number of rings to be: optic tectum layer 3, 1.34 X 10(2)/mm(3), layer 5, 1.14 X 10(2)/MM(3), nucleus prerotundus 4.63 X 10(2)/mm(3), olfactory bulbs, 6.2 X 10(2)/mm3. In the brains of fish kept at 15 degrees C a few rings are found only in some olfactory bulbs and not in any other region of the brain. The brain wet weight also changes and was found to be significantly (p less than 0.01) less in 5 degrees C than in 15 degrees C fish, the mean values being 0.1588 g and 0.2091 g respectively. The significance of the observed changes is discussed. It is suggested that the smaller brain wet weight may reflect a change in the vasculature of the brain and be related to the acclimation process. It is hypothesized that the rings are a morphological expression of a physiological change caused by prolonged exposure to low temperatures; a neuronal response reflecting either early degeneration or a functional adaptation which may involve an alteration in axoplasmic transport. It is suggested that this system is a useful model for the study of relationships between neurofibrillar disturbances and neuronal function.