After World War II he worked in the laboratory of Prof. UG Bijlsma in the area of adrenergic substances and in 1950 both in the field of chemistry and medical doctorate. In 1951, Everhardus Ariëns moved to Nijmegen after there at the Catholic University of the Faculty of Pharmacology was established. From 1954 until his retirement he was employed there as a professor.
Based on his dissertation, he developed together with Jacques van Rossum, a method for quantification of pharmacological effects as a result of ligand-receptor interactions. The thesis developed the concepts of receptor affinity and intrinsic activity. With the help of these terms he could describe the behavior of agonists and antagonists as well as the dual agonist / antagonist behavior of partial agonists. An important accomplishment of Ariëns was the establishment of experiments on isolated organs instead of the living animal (ex vivo), which quickly and reproducibly delivered data on the affinity and intrinsic activity of test substances.
Everhardus Ariëns was also active in the field of structure-activity relationships (SAR), a branch of medicinal chemistry. With the provocative statement that the then commonly used racemates were drugs with 50% contamination. he triggered a debate among pharmacologists and medicinal chemists and alerted the drug regulators. Everhardus Ariëns was thus the crucial precursor for the targeted development of enantiopure drugs. Another, though less noticed controversy he started by expressing his view that drug metabolism is wasted and called for the development of metabolism-resistant drugs. In addition, he followed in the tradition of Dutch pharmacists to combat quackery.