Male adult rabbits and rats treated with Spartium junceum showed a significant decrease in fertility, demonstrated by a lower number of pregnancies. The target of the drug seems to be the acrosomal protease system, the activity of which appears greatly reduced, while the morphology of testicular cells and epididymal spermatozoa is only partially affected. The antifertility effect is completely reversible.

Intraperitoneal injections of extracts of Spartium junceum in adult male rats were shown to reduce the rate of fertility and acrosin enzyme activity. For the latter the drug effects could be divided into two phases: in the first stage a significant increase in proteolytic activity was observed in all experimental animals examined; and in the second stage, there was a general decrease of up to 50% in enzyme hydrolytic properties. There is a notable difficulty in correlating the acrosin activity and the data obtained from mating tests. The effect of drugs on the activity of another acrosomal protease, benzamidine-resistant protease, was assayed. A significant decrease in this enyme was noted though the modality was different from that of acrosin. Extrapolating the data of the two co-extracted acrosomal proteases, the ratio of acrosin-like protease [E] and benzamidine-resistant protease [E]x, it was possible to correlate the value expressed to the rate of pregnancy. The implications of this finding is discussed although the exact molecular basis of the phenomenon is uncertain.