Determination of the peroxides formed during blowing with air or oxygen at room temperature has been regarded as the best method for testing the stability of fat oils, and especially for determination of their induction periods. The present investigations are chiefly dealing with airblowing of medicinal cod liver oils, to find if a more rapid stability test can be obtained for these oils by elevation of the reaction temperature. The results can be summarized as follows: 1. The oils were treated with air in an apparatus permitting controlled temperature and airstream conditions (fig. 1). 2. No reaction products with prooxidativ effect were blown off the oils with the airstream at a temperature of 40° C. Diffuse daylight, the humidity of the air, or filtering of the oils, were all factors of minor importance. 3. Increase in the airstream increased the rate of peroxidation considerably, but to a very different degree in different cod liver oils. In relation to calculated zero peroxide numbers (airstream->O), the blowing with 50 l-air per hour at 40° C through 30 g oil, increased the rate of peroxidation from 50,5 to 113,5 °/o. The stability test therefore must be carried out with minimum airstream. 5 l air per hour through 30 g oil was found to give relatively small errors, and at the same time sufficient stirring of the oils. 4. For resistent oils the absorbtion of oxygen per hour increased with the time. Destearinized oils, on the other hand, showed no induction period, and a slight decrease in the rate of peroxidation with the time took place. 5. The temperature coefficients of the resistant oils showed no relation to the temperature or the blowing time, but changed extremely irregular. For the destearinized oils these coefficients showed considerable constancy, averaging 1,71 from 19° C and upwards to 70° C. 6. The reliable stability relations of the steam rendered cod liver oils, expressed as quotients of the increase in their peroxide numbers after airblowing at 20° C for 72 hours, equalled the stability relations found by blowing at 60° C for 3 hours. This was especially true for the destearinized oils. No other blowing time at any elevated temperatures up to 70° C gave these stability relations. 7. By destearination of the oils the antioxidants were filtered off with the stearin - this -was also the case with the bulk of the oil components containing phosphorus. This indicates that the antioxidants in unstearinized cod liver oils are mainly phosphatides. 8. The antioxidants in unstearinized oils were destroyed in about 1/2 hour by 100° C. 9. Washing with destilled water increased the stability. 10. In a liver oil, stored airtight for half a year, the decomposition of peroxides and the formation of epihydrinaldehyd (Kreistest) approximately paralleled each other. 11. Unstearinized oils, produced by centrifugal methods from the fresh residues of steamheated cod livers, had much lower resistance to oxidation than the corresponding steam rendered oils.