The cardiopulmonary effects of HI-6, together with atropine and soman, were studied in the rat. HI-6 is an effective antidote in acute poisoning with the nerve agent soman. The therapeutic efficiency of HI-6 is still unclear and cannot be explained entirely by the HI-6 reactivating ability of acetylcholinesterase (AChE). Other non-cholinergic factors must be involved. One possible detoxifying process might be an effect of HI-6 on the blood flow to sensitive organs. The purpose of the present study was to investigate 1) whether soman per se induces changes in regional blood flow and 2) whether the blood flow to different organs is affected when HI-6 (50 mg x kg(-1) i.m.) and atropine (10 mg x kg(-1) i.m.) are given either before or immediately after soman intoxication (90 microg x kg(-1) s.c.). For regional blood flow determinations the microsphere method was used with male Wistar rats weighing 300-400 g. The rats were anaesthetised and breathed spontaneously during the experiment. Three different blood flow measurements were made in the same animal and concomitant physiological parameters such as mean arterial blood pressure and respiratory rate were recorded. The blood AChE activity was followed throughout the experiment. Our results show that when HI-6 is given after intoxication with soman, dramatic changes in blood flow occur with a significant decrease in both respiratory rate and blood AChE activity. If HI-6 is given prior to the intoxication, however, all rats are unaffected and none of the parameters measured are changed.