Growth of cotyledons excised from 5-day-old dark grown cucumber seedlings was measured after certain wounding treatments. Cotyledons rubbed between wet thumb and finger with or without carborundum or cut into eight sections grew faster than non-rubbed, non-cut controls. To determine how such treatments promoted growth, scanning electron micrographs of cotyledon surfaces and growth responses in H2O, zeatin, KCl and ethylene were evaluated. Treatments enhanced growth without zeatin or KCl, but were more effective when KCl was present. Abrasion with carborundum caused tears in cotyledon surfaces and faster absorption of KCl. Nevertheless, growth (H2O absorption) in KCl was so much greater that tissue K+ concentrations after growth were lower in abraded than in nonrubbed control cotyledons. Increased permeability of zeatin, KCl, H2O or O2 is unlikely to explain most of the observed growth enhancement, because such enhancement also occured when only the adaxial, astomatal surface was abraded and placed either down (in contact with the growth medium) or up (in contact with air). Effects of exogenous ethylene indicate that this gas does not cause growth enhancement. Perhaps an unidentified factor produced in wounded cells increases growth.