LEONARD & JlbRGENSEN awareness in experimentalists of the possibility of designing experiments for and subjecting them to more quantitative and penetrating analyses. Certain aspects of transcapillary transport in capillary beds have been reviewed previously (5--10) but this is, so far as the authors know, the first attempt to review convection and diffusion in capillary beds as a single subject. We know that the results are incomplete. We have eschewed the subject of compartmental analysis except when it seemed to be applied in detail to capillary beds. We are not competent to provide a physiological review of the response of capillary beds to drugs. We have not done justice to the beginnings of a proper analysis of solvent and protein convection in tissue nor to the detailed analysis of interesting special vascular structures such as those found in the kidney, placentas, and other rete-like arrangements. These omissions signify our limitations of time and ability, not our assessment of their importance nor their relevance to what is reviewed.