I HAVE for some years been investigating the action of drugs on mammalian spermatozoa, under the auspices of the Birth Control Investigation Committee. Finding that certain drugs seemed to stimulate sperms to higher activity, I have recently made a special study of this problem. I have used guinea-pig sperms taken from the epididymis and suspended in a glucose-saline fluid buffered at about pH 8. Sperms are so active when first suspended in this fluid that the effect of stimulating drugs is not obvious. In these experiments the sperm suspensions were allowed to remain for 5–8 hours at the temperature of the body before the addition of the drug. After 5–8 hours, the activity of the sperms was markedly reduced. The drugs were tested at concentrations in the series 1,1/4, 1/16, 1/64 per cent, etc.