The relative amounts of the different forms of morphine, and many other pharmacologic agents, depend on temperature and pH. Some forms are more efficacious because they are uncharged and can penetrate lipid membranes more easily than the charged forms. Persons who administer pharmacologic agents to ectotherms (that is, cold-blooded animals) should consider the effect of temperature on the relative amounts of the different forms of drugs. For example, the fraction of morphine present in the uncharged form is twice as high in a fish or frog at 5 degrees C as in a mammal at 37 degrees C. Moreover, because the pH of blood, plasma, and tissues of ectotherms is higher when they are held at lower temperatures, the combined effect of temperature and pH on the speciation of pharmacologic agents also should be considered. In addition, the total solubility of morphine and other pharmacologic agents depends on temperature and pH. The purpose of this overview is to describe how temperature and pH influence the solubility and speciation of morphine.