The effects on behaviour of single subtoxic doses of two potent organophosphorous compounds, sarin (isopropyl methylphosphonofluoridate, 12.5 and 50 micrograms/kg, intraperitoneally) and soman (pinacolyl methylphosphonofluoridate, 4 and 20 micrograms/kg, intraperitoneally) were studied in male Wistar rats. In the open field test, soman dose-dependently decreased rearing and ambulation and increased non-mobile exploration. The higher dose of sarin changed only the rearing and grooming behaviour. Sarin and soman decreased locomotor activity on the Animex for at least one hour at the beginning of the monitoring period. In the doses used, both organophosphates inhibited acetylcholinesterase (AChE) activity significantly in the blood. The results suggest that small doses of sarin and soman have inactivating effects on the behaviour of rats. Although the findings cannot be extrapolated directly to behavioural changes in man, they indicate that subtle behavioural dysfunctions could also occur in humans at exposures which do not cause acute toxicity.