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As government regulations for animal care increase in number and complexity, and as animal-rights activists continue to push for decreased use of animals for research, more laboratories have turned to tissue and cell culture for biological research. Initial costs of large animals and escalating maintenance costs have driven some researchers from the use of large animal models. Both models-whole animals for chronic experiments and the use of isolated tissue-can give answers to physiological questions. Cost is certainly a factor that must be considered in the present atmosphere in which funding is so difficult to obtain. However, any information gained in isolated tissue must eventually be assessed in the whole animal, where many factors interact to control physiological mechanisms.