Physiological state of dairy animals is a predisposing factor in environmental influences on animal health. Critical phases of life cycle include neonatal period, postpubertal reproduction, and lactation. Primary effect of environment in neonatal period is increased disease incidence associated with reduced immunoglobulin content in plasma of calves. Cold stress has little effect on reproduction; in contrast, heat stress reduces libido, fertility, and embryonic survival in cattle. Heat stress in late gestation reduces fetal growth and alters endocrine status of the dam. Carryover effects of heat stress during late gestation on postpartum lactation and reproduction also are detectable. Heat stress of lactating cattle results in dramatic reductions in roughage intake and rumination. Decreases in roughage intake contribute to decreased volatile fatty acid production and may contribute to alteration in ratio of acetate/propionate. Rumen pH also declines during thermal stress. Electrolyte concentrations, in particular sodium and potassium, also are reduced in rumen fluid of heat stressed cattle. The decrease in sodium and potassium are related to increases in loss of urinary sodium and loss of skin potassium as well as decline in plasma aldosterone and increase in plasma prolactin. Reduction in thyroxine, growth hormone, and glucocorticoid concentrations in chronically heat stressed cattle appear to be related to decreases in basal metabolism.