Chickens vaccinated with herpesvirus of turkey (HVT) as 18-day embryos or at hatching were challenged as neonates with pathogenic Marek's disease (MD) virus (MDV). Embryonally vaccinated chickens had much greater resistant to challenge than chickens vaccinated post-hatch. Embryos became readily infected with HVT regardless of whether the vaccine was deposited into the body of the embryo or extraembryonally, such as in the amniotic sac. Embryonally vaccinated chickens were viremic with HVT at hatching and remained persistently viremic through the duration of the experiment. The titer of recoverable virus was higher in the embryonally vaccinated chickens than in the chickens vaccinated post-hatch. Embryonal vaccination did not affect hatchability. Vaccination at any stage of embryonation tested protected better against neonatal challenge than did vaccination at hatching. Protection against an early challenge was greatest when the embryos were 17 or 18 days old at the time of vaccination. Lower protection in chickens vaccinated as 11-day embryos was not due to humoral immunologic tolerance. Chickens vaccinated at the 11th day of embryonation were poorly protected against MDV challenge at three or eight days of age but were well protected if the challenge was delayed until the 14th day of age.