An attempt was made to upset the equilibrium between mutation pressure and selection pressure in a natural population of deer-mice (Peromyscus maniculatus) through irradiation of sexually mature males. A population was kept under observation by use of nest boxes. After one year of observation exposure of males to a 500 r gonadal dose of x ray was begun and continued for a five-year period. Certain aspects of population dynamics were highly pertinent to the conduct of the experiment and to the results obtained. Deer-mice are primarily winter breeders in the region studied. Belatively permanent mating pairs are formed. The average dispersal distance of the young is short. The life span is short and population turnover high, indicating high environmental pressure on the population. Reduction in litter size is attributed to radiation-induced lethal mutations. Maintenance of population numbers through four years of irradiation is attributed to high selection pressure, and a consequently high critical mutation rate in a species such as this.