Insect herbivory has variable effects on plant physiology; so greater understanding is needed about how injury alters photosynthesis on individual injured and uninjured leaves. Gas exchange and light-adapted leaf chlorophyll fluorescence measurements were collected from uninjured and mechanical partial leaf defoliation in two experiments with Nerium oleander (Apocynaceae) leaves, and one experiment with Danaus plexippus herbivory on Asclepias curassavica (Asclepiadaceae) leaves. Gas exchange impairment (lower photosynthetic rate (Pn), stomatal conductance (gs)) indicates water stress in a leaf, suggests stomatal limitations causing injury Pn impairment. The same pattern of gas exchange impairment also occurred on uninjured leaves opposite from injured leaves in both N. oleander experiments. This is an interesting result because photosynthetic impairment is rarely reported on injured leaves near injured leaves. No photosynthetic changes occurred in uninjured A. curassavica leaves opposite from D. plexippus-fed leaves. Partially defoliated leaves that had Pn and gs reductions lacked any significant changes in intercellular leaf [CO2], Ci. These results neither support, nor are sufficient to reject, stomatal limitations to photosynthesis. Manually imposed midrib vein severance in N. oleander experiment #1 significantly increased leaf Ci, indicating mesophyll limitations to photosynthesis. Maximal light-adapted leaf photochemical efficiency (F'V/F'M) and also non-photochemical quenching (qN) were reduced by mechanical or insect herbivory to both study species, suggesting leaf trouble handling excess light energy not used for photochemistry. Midrib injury on N. oleander leaves and D. plexippus herbivory on A. curassavica leaves also reduced effective quantum yield (ΦPSII) and photochemical quenching (qP); so reduced plastoquinone pools could lead to additional PSII reaction center closure.