There has been a great deal of research concerning the heart being an important regulator of renal fluid and electrolyte excretion. This cardiac-renal connection involves two different types of major mechanisms, both of which are covered in this review. The first of these to be discovered was neural reflex regulation. This type of control is due to the fact that the heart possesses nerve receptors whose activity is altered by changes in the degree of cardiac stretch that occur as a result of changes in blood volume. These receptors affect various humoral, neural, and perhaps hemodynamic mechanisms that modify renal excretion. A second, more recently discovered type of regulation is based on the concept that the heart is also an endocrine gland. Similar to neural receptor activity, cardiac hormone secretion is also linked to the degree of cardiac stretch or filling. These cardiac peptides have been shown to have a variety of physiologic effects, most of which directly or indirectly affect renal excretion. Both of the above cardiorenal control mechanisms, one neural and one humoral, may be important not only in maintaining normal fluid-electrolyte balance but may also have pathophysiologic relevance.