Kostial began working in the medical faculty at Institute for Medical Research and Occupational Health as an assistant in 1950, while she continued her graduate studies.[3] Taking courses in the Department of Physiology at the University of Zagreb in 1951 and 1952, she received a World Health Organization (WHO) Scholarship to work as a research assistant at University College London. Completing her research in 1953, Kostial returned to the University of Zagreb and earned her Doctorate of Medical Sciences degree in 1955 and her Habilitation in Physiology the following year.[2]