The self-induced inactivation of antheridiol is a natural hormonal effect specific to Achlya species, as is demonstrated by its occurrence in homothallic cultures of A. americana in association with the formation of sex organs. We have also shown that progesterone and a biologically inactive isomer of antheridiol do not induce their own metabolism, and that water moulds that do not respond sexually to antheridiol are also not induced to metabolise it.