Effects of phthalic acid monoesters on mouse testes have been studied. Testosterone concentrations in the testes on mice fed diets containing 2% of mono-n-butyl (MBP), mono-iso-butyl (MIBP), mono-2-ethylhexyl (MEHP) or mono-n-octyl phthalate (MOP) were lower than in control animals and relative testicular weights were increased in all mice except those which received MOP. Zinc concentration in the testes decreased in mice that received MBP, MIBP or MEHP.