A single injn. of 5 mg. of testosterone propionate in the form of macrocrystals in methyl cellulose or in an emulsion stimulates the seminal vesicles and ventral prostate glands of castrate immature rats to a greater extent and for a longer period of time than does a single injn. of testosterone (free or the propionate) adsorbed on aluminum phosphate. All of the prepns. tested are much longer lasting in their effects than is the commonly used soln. of testosterone propionate in oil.