This report deals with the radioactivity of the blood. and the spermatozoa of the cock after subcutaneous or intravenous injection of labelled phosphate (P32). The maximum radioactivity of the blood of the cock reached 15 minutes after the injection of labelled phosphate. Then the activity of the blood decreased rapidly within 2 to 3 hours after injection, and it was found that the activity began increase after about 3 days and kept comparatively high level for the long days after the lapse of 5 days. The radioactivity of the spermatozoa in the ejaculates began increase after the lapse of 2 weeks and reached the peak after about 3 weeks and then decreased gradually. From these results it may be estimated that the duration of spermatogenesis of the cock may be about 1 week and the mature spermatozoa formed in the testis will be ejaculated 2 weeks afterward.