In a poem setting forth the way things are in nature, it is fitting for Lucretius to address, among many other phenomena, human conception and embryonic determination. With an eye toward ethics, Lucretius demonstrates how sexual reproduction at the seminal level can be explained by Epicurean atomism. In this paper, I am concerned with the biological ‘how’ of conception as explained inDe Rerum Natura(=DRN) but also with the ethical ‘therefore’ for Lucretius’ readership and (over)estimations of male autonomy. For modern audiences (...) with a basic grasp of procreation that includes sperm supplied by a male and egg supplied by a female, encountering Lucretius’ verses on women contributing semen (semina) to the process of conception can be surprising (4.1209–62). The idea of female semen may give us pause as we calibrate it with our understanding of eggs and ovulation, but Lucretius, in his time, was not advancing some novel theory. Wading into established debates on male-only or joint male-female semen production and gendered insemination (that is, who produces semen and whose semen is active at conception), Lucretius sides with those promulgating mutuality for both questions (for example Democritus [DK 24 A13]) and rejects Aristotle's representative exclusivist claim of male activity vs female passivity (τὸ ἄρρεν ἐστὶν ὡς κινοῦν καὶ ποιοῦν, τὸ δὲ θῆλυ ὡς παθητικόν,Gen. an.729a28–30; cf. 726a30–6). That is to say, a sexually mature female, like her male counterpart, emits semen that has determining potency in the formation of a human embryo (Lucr. 4.1209–62). Although the discharge and activity of female semen is the focus of this paper, my investigation is not aQuellenforschungor historical survey of Greco-Roman ideas about women's contributions to insemination and fertility, since others have treated these matters extensively. I concentrate rather on how Lucretius employs the concept of female semen in terms of his poetics in Book 4 and what I see as an ethical argument against the domineering nature of Roman masculinity. The problem of female semen, from the point of view of Lucretius’ Roman male audience, is that it is potentially costly to men because it rivals and threatens their status from the physiological to the discursive level. Iain Lonie broaches the same issue from Greek perspectives. (shrink)