The effects of acute choline (Ch) administration on the hot plate (HP) analgesia produced by soman and by morphine and on the toxicity produced by soman were examined in male rats. Morphine (9.0 mg/kg, IP) and soman (80 micrograms/kg, SC), but not Ch (60 mg/kg, IP), produced HP analgesia. Pretreatment with Ch (100 mg/kg, IP; given 20 min earlier) reduced morphine and soman analgesia by 68% and 38% respectively. Ch (LD50 = 405.8 mg/kg, IP) at doses above 200 mg/kg rapidly produced signs of cholinergic stimulation which disappeared by one hr. A fixed dose of Ch (100 mg/kg) altered neither the expression of toxic cholinergic signs produced by varying doses (from 75.9 to 151.4 micrograms/kg) of soman, nor the 24-hr LD50 (124.0 micrograms/kg, SC) of soman. Doses of Ch ranging from 213.8 to 269.2 mg/kg when given 40 min prior to a fixed dose (93.3 micrograms/kg) of soman resulted in persistent signs of cholinergic hyperstimulation, but again, did not affect the mortality produced by soman. The results demonstrate that acute Ch pretreatment increased the severity of cholinergic stimulation and reduced the HP analgesia, but were not accompanied by potentiation or antagonism of the lethal action of soman.