Ejaculation is “the emission of semen to the exterior, resulting from the contraction of muscles surrounding the male internal urogenital ducts” (Medline’s definition). It is closely connected to the male orgasm. Sexual behaviour has components of rewarding behaviour, and ejaculation appears to be the most reinforcing component. Little is known about the neurochemical basis of this rewarding experience. Some relations to opioids and their receptors have been shown (Coolen et al. 2004).