Two groups of each 8 adult male hooded rats of the Long-Evans strain received single subcutaneous injections of 1 microgram/kg arginine-vasopressin (AVP) or 1 microgram/kg lysine-vasopressin (LVP) 15 min before investigation in an open field (OF) and 30 min before investigation of locomotion in a wheel (W). The behavioural parameters were statistically compared with two control groups. AVP significantly increased infrared beam crossings in the OF, wheel-running and the mobility of the rats in both tests. LVP had a strong just opposite effect. LVP facilitated habituation and AVP weakened it. AVP facilitated rearing, whereas LVP suppressed it. The duration of immobility was strongly enhanced after LVP in both tests (380% in the OF and 920% in the W) but AVP had no prolonging effect. The latency of the first immobility phase in the OF was increased to 250% after AVP and decreased to 45% after LVP. Grooming was not influenced by AVP, whereas LVP strongly reduced the grooming duration and prolonged the grooming latency in the OF. The contrary effects of AVP and LVP are highly significant. They may be summarized as activating effect on motor behaviour and exploration by AVP and inhibitory action of LVP.