A great deal of evidence has accumulated in recent years to suggest that th
ere has been a gradual increase in male reproductive pathology over the pas
t 30-40 years, as evidenced by increased rates of testicular cancer and dec
lining semen quality. The hypothesis is advanced that this phenomenon is ca
usally related to the ability of male germ cells to generate reactive oxyge
n metabolites. When produced in low levels, such metabolites are thought to
enhance sperm function by stimulating DNA compaction and promoting a redox
-regulated cAMP-mediated pathway that is central to the induction of sperm
capacitation. When produced in excessive amounts, the same metabolites stim
ulate DNA fragmentation and a loss of sperm function associated with peroxi
dative damage to the sperm plasma membrane. Free radical-induced mutations
in the male germ line may also be involved in the aetiology of childhood ca
ncer and recent increases in the incidence of seminoma. Ln light of these c
onsiderations, establishing the mechanisms for free radical generation by t
he male germ line and determining the factors that influence this activity
are important objectives for future research in this area.