AbstractAn investigation has been made into the effect upon meteorites in space of the primary rays in the exceedingly strong cosmic radiation that falls upon these bodies before they penetrate the Earth's atmosphere. Specimens of meteorites with terrestrial ages (i.e., intervals since time of fall) ranging from 9 months to 457 years were studied by means of the nuclear‐emulsion‐plate technique.The oldest meteorite studied (Ensisheim, Alsace, France) was found to be inactive, and the next oldest (Pitts, Georgia) was only very slightly active. On the other hand, the relatively young Walters, Oklahoma, meteorite showed considerable activity, and a metallic inclusion and one piece of crust from the very recent Norton‐Furnas Counties, Kansas‐Nebraska, achondritic fall of 1948 February 18 were very active indeed. Another piece of Norton crust showed only slight activity, and two other fragments of this achondrite were inactive.Examination of the nuclear “stars” recorded on the emulsion plates shows that the activity observed in the specimens studied is an induced one and cannot be attributed to the low content of natural radioactive elements present in the meteorites. Tentative explanations of this induced activity are advanced i