Anaesthesia by cooling has been applied to neonatal rats (Wiesner, 1934, and others) and mice (Woolley & Little, 1945) but few details of technique or survival rates have been given. We therefore report a technique for anaesthesia that has been used successfully for the castration, thymectomy, unilateral nephrectomy and splenectomy of C3H, AKR and T.O. mice up to the age of 2 days. The following schedule is common to all operations. The mother is gently persuaded to leave her litter and some of the babies are removed from the nest. They are then transferred in individual, dry, plastic, ice-cube containers to the freezing compartment of a domestic refrigerator. The cooling time required (6–12 min.) varies with strain, body weight and temperature of the compartment. A mouse is fully anaesthetized when it has ceased to breathe, does not respond to stimulation and has lost all pink colour. The operation should be