This paper describes spatio-temporal variation in gas exchange over the lower surface of primary leaves of glasshouse-grown Phaseolus vulgaris L. plants. Simultaneous measurements of assimilation and water vapour conductance were made with a small area cuvette attached to an infra-red gas analyser. The plants were kept in glass chambers so that the external gaseous environment could be controlled. Observations are reported from four half-hour periods during a day in which the ambient PPFD, while variable, was close to saturating for photosynthesis. ‘Snapshot’ measurements of gas exchange were made at 20 positions on the leaf surface using a stratified random out-to-in strategy, which avoided disturbance of yet-to-be-measured sites. Data were mapped using the ‘Unimap’ cartographic program. For any given measurement period, gas exchange varied greatly over the leaf surface: typically, net assimilation (A) varied by over 4-fold and leaf conductance (gl) by over 3-fold. Estimated intracellular pressures of CO2 and leaf temperatures showed less relative variation both in space and time. Comparing measurement periods, the spatial patterns of variation in A and gl were dissimilar. Moreover, at different sites on the leaf, the trends in a given variable could be in opposite directions, while external conditions were relatively constant. Although the correlation between A and gl was significant overall, there was a large degree of scatter in the data and zones of high gl often corresponded to areas of low A. Depending on the basis of calculation, A was as much as 63% lower than a value predicted on the basis of steady-state measurements for the appropriate mean irradiance. It was not possible to deduce from the relationships between pairs of variables which factors were most important in determining A and gl at any given time or space, but gl did not appear to be the only factor limiting A. It is hypothesized that the observed variation in gas exchange, the lack of close correspondence between gl and A and the reduction in photosynthesis compared with the apparent potential value are all phenomena that arise from differences in the induction times for these variables following changes in conditions, interacting with other factors associated with position on the leaf.