Excessive amounts of heavy metals (HMs) in the rooting media affect adversely plant growth, cause loss of yields, and undesirable effects on crops. The aim of this study was to investigate the effects of exogenous sodium nitroprusside (SNP) (200 mu M), a nitric oxide (NO) donor, on toxic cobalt (Co) level (200 mu M) in iceberg lettuce (Lactuca sativa L. var. capitata). Plants were grown in a hydroponic system (modified Hoagland solution and perlite as inert media) inside a greenhouse under natural light conditions (average temperature 27-18 degrees C day-night and humidity 63%). Results indicated that excessive Co caused significant reductions in fresh (FW) and dry weights (DW) of the shoots and roots, photosynthetic pigment contents of leaves, and metallic cation concentration of shoots and roots. Also, hydrogen peroxide (H2O2), lipid peroxidation, proline accumulation, catalase (CAT) and ascorbate peroxidase (APX) activities, and total Co accumulation rate (TAR) increased significantly with excessive Co. Compared to Co application, Co+SNP application showed a 48.5% and 31.1% increase in FW and DWs, respectively. Moreover, chlorophyll (Chl) a, Chl b, Chl a+b and carotenoid (Car) content increased 54.5%, 64.4%, 56.6%, and 39.7%, respectively. Similarly, shoot Co uptake (69.7%), shoot and root Fe concentration (21.7% and 149.4%, respectively), root Zn concentration (6.1%), and net accumulation of Fe and Mn via roots (4.5- and 1.6-fold, respectively) were increased. Consequently, the alleviating effect of exogenous NO on Co toxicity in iceberg lettuce could depend on NO-induced increase in antioxidant enzyme activity and its multifunctional role in plant cellular mechanism.