Under favourable conditions tetanus spores may germinate and multiply in vivo soon after their introduction into the tissues. Dorothy Russel [3] found that vegetation was visible microscopically on the second day in necrotic foci produced in guinea-pigs by the injection of earth. In lesions produced by 5% calcium chloride solution considerable vegation of Clostridium tetani could be seen at 24 hours. Fildes [1] observed vegetation as soon as 2 hours after the injection of spores into the ligated testicle in guinea-pigs. The present study describes a quantitive investigation of the growth of Cl. tetani in vivo, and the findings are related to formation of tetanus toxin in vivo.