Recent experiments which demonstrate the limited and complementary photosynthetic capacity of mesophyll and bundle sheath chloroplasts in the leaves of C4 plants emphasize the role of structural organization in this photosynthetic process (Slack, Hatch, and Goodchild 1969; Edwards et al. 1970; Woo et al. 1970). The current interpretation of C4 photosynthesis involves the cooperative activity of chloroplasts in both cell layers and demands the transport of photosynthetic metabolites between tissues during ,C02 fixation (Karpilov 1969; Downton 1970; Hatch and Slack 1970). This paper examines the rates of organic anion transport implied by this hypothesis, demonstrates the rapid appearance of label in bundle sheath cells during 14C02 fixation, and discusses structural aspects of the system relevant to transport.