Further development of conceptual elements of the theory of self-care, one of the three constituent theories of Orem's self-care deficit theory of nursing, is reported. Five content areas of a practical science of self-care are identified; one content area, self-care requisites, is refined and developed. The nature of self-care requisites is reformulated; guides and standards for the expression of self-care requisites, examples of expressed self-care requisites, and a self-care practice guide are described. These developments are illustrated using the example of the requisite to maintain an adequate fluid intake.