The inducibility of heritable mutations in female mammals has been measured in the mouse specific-locus test (SLT). For radiation-induced mutations, a large body of data has been accumulated. However, relatively few SLT studies in females have been conducted with chemicals. Of only 5 chemicals so far explored for their effect in oocytes, two, ethylnitrosourea (ENU) and triethylenemelamine (TEM), and possibly a third, procarbazine hydrochloride (PRC), are mutagenic with at least one of these (ENU) mutagenic in arrested as well as maturing oocytes. However, the mutation rate is, in each case, lower than for treated male germ cells. By contrast, ENU-induced mutation yield for the maternal genome of the zygote is an order of magnitude higher than that for the zygote's paternal genome or for spermatogonia. A high proportion of mutants derived from chemical treatment of oocytes are mosaics, probably owing to lesions affecting only one strand of the DNA.