Infection and tumors provoke substantial changes accompanied with the disbalance of many neuroendocrine factors which in their summarizing effects influence the life span of animals. Our previous results showed enhanced mortality after one injection of morphine in association with Friend leukaemia virus infection. The aim of this study was to examine the effects of some other opioids (pethidine and pentazocine) and an acetylcholine esterase inhibitor neostigmine on the survival of animals under two conditions: (1) Friend leukaemia virus infection which mostly depressed immune functions, and (2) Toxoplasma gondii infection which in general enhanced the immune status. In contrast to our previous observation with morphine, the mortality induced by single doses of pethidine (150 mg/kg) or pentazocine (50-75 mg/kg) was unchanged during the Friend leukaemia virus infection. A single injection of neostigmine (0.42 or 0.56 mg/kg) was significantly more lethal in DBA-2 mice infected with Friend leukaemia virus. Neostigmine in doses of 0.33 and 0.4 mg/kg caused death in 46 % and 57 %, respectively, of animals infected with Toxoplasma gondii which was significantly higher in comparison with only 8 % and 12.5 % in control groups. Pethidine (150 mg/kg) killed 70 % of Toxoplasma gondii infected animals and even 90 % of non-infected mice. Thus, the Friend leukaemia virus and Toxoplasma gondii infections increased toxicity only of some drugs which may, at least partly, be associated with altered immune status during infection and involvement of the cholinergic system.