Abstract: Opioid system is one of the systems to modulate physiological activities in the body. According to the investigations in recent decades, opioids are presumed to play a vital role in the control of growth in farm animals and other species. However little information is available regarding the effects of opioids on tissue weights and DNA, RNA and protein levels of tissues in farm animals. Effects of opioids on macromolecular constituents of tissues in guinea pigs, opossums and salamanders are described in the article. Although it is unclear whether opioids could affect the function of tissues or not, opioids can induce immunosuppression in mice and humans.