In a colony of Kyo: Wistar strain of rats, animals were found with curled whiskers and hair. These rats exhibited tremor when they moved. There were no sex differences in phenotype and behavior, and the affected animals of both sexes were sterile. Among pairs that produced at least one tremorous offspring, 21.8% of the females and 21.7% of the males were affected; these proportions suggest that the anomaly is caused by an autosomal recessive gene. When the presumed heterozygous males were crossed with WAG/Rij females, about half of their female F1 hybrids were heterozygous, and they produced 26.1% (43/165) of the affected offspring when backcrossed to the sires. The disorder may be cause by an autosomal recessive gene. The gene is tentatively designated as tremor (tm). The main pathological changes were seen in the gonad and CNS. The gonads of both sexes were aplastic even in adult animals. Vacuole formation was seen widely in the central nervous system and sometimes exhibited a spongy appearance. After administration of .alpha.-methyl-p-tyrosine, the norepinephrine content of the cerebellum was high, indicating that some anomalies of catecholamine release were present. The mutation is being maintained by random mating of the littermates of affected animals. Detailed pathological, endocrinological, neuropharmacological, and genetical studies are proceeding.