Mortality of non-littermate male rats joined in parabiosis at 31 days of age from parabiosis intoxication was 64%; signs of intoxication followed by recovery were shown by 28%, and 8% remained asymptomatic. Treatment with an antihistaminic or cortisone had no effect on the incidence of severity of the intoxication. Litter-mate pairs showed less tendency toward intoxication, but those which were affected died as readily as non-littermates.