There is now substantial evidence that chemical regulation of shoot physiology occurs in droughted plants in the field. The evidence that ABA may play a role in such regulation is considered, and topics of relevance to the worker interested in determining the ABA relations of plants in the field; such as the methods used for ABA quantification, the relevance of quantifying ABA in various plant tissues, methods of xylem sap collection and timing of sap collection are reviewed. A possible role of tissue sensitivity to ABA in controlling the diurnal changes in stomatal conductance and leaf growth rate seen in the field is also considered.