1. after 80 serial passages from brain to brain of chicks, neurotropic virus fails to show essential modifications in the behaviour of the virus in mice or chicks. 2. chicks are susceptible by intracerebral, intraperitoneal and intradermal route, showing high levels of circulating virus and become immunes when inoculated during the first days after birth. but, circulating virus varies, however, in inverse ratio to age. it does not seem possible to infect chicks by gastric route. 3. during first days virus may be found, occasionally in the lungs, liver, spleen and kidneys; some days afterwards, no chicks showed virus in other organs than brain, in which virus persists up to 10th day post-intraperitoneal inoculation and up to 15th day after the intracerebral inoculation, and perhaps till later. we have however, not been able to isolate the virus from excretions. 4. there does not seem to be any difference in susceptibillity to the yellow fever virus, on chicks with b avitaminosis. 5. detectable immune bodies appear by 10-11 day after intraperitoneal and intracerebral inoculation, at time, that many of some chicks still have virus in the brain. 6. age has a marked influence on the development of immunity, in chicks inoculated by intraperitoneal route. 7. the multiplication and circulation of virus after intradermal inoculation of 50 to 160 m.l.d., renders possible to suggest that infected mosquitoes can difuse the virus among chicks and, perhaps, to other birds, a few days old.