In experiments on mice with the tonic pain caused by intro- duced of 5 % of formalin in hindpaw, are quantitatively measure changes of painful behavioural reaction (PBR) values under action low intensity electromagnetic field (EMF) and sub- stances that modulate activity of NO-synthase system, and at both of these action also. Such substances were used: in qual- ity inhibitor NO-synthase - Nw-nitro-L-arginin (L-NAME), donors NO - sodium nitroprusside (SNP) and sodium nitrite (SN). Influence of the EMF completely eliminated effect of the L-NAME and SN in concentration of 0.5 mM/l, and also SNP in concentration 5 mcM/l on the painful reaction. The assumption that neurogenetics and NO- ergic mechanisms are basis of the systems analgesic effects of the low intensity electromagnetic fields. Mechanisms of modulation of painful behavioural reaction and other behavioural displays of animals under condition EMF and NO are discussed.