The study of nicotine action upon N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) stomach cancer development was carried out in rats. As compared with control it was established that combined MNNG and nicotine long-term administration led to the occurrences: 1) stomach pretumorous changes of the whole mucous membrane; 2) earlier development of stomach cancer tumors and its frequency was doubled; 3) progressive decrease of acetylcholinesterase activity, especially expressed in homogenates of cerebrum hemispheres, hypothalamic region and medulla oblongata (where this activity is practically failed to be expressed). The obtained data are the witness of the nicotine ability to enhance the MNNG-stomach carcinogenesis in rats. The possible action of the autonomic nervous system in such nicotine effects is under discussion.