Purpose. Extraocular muscle (EOM) pulleys are important in eye movement physiology and may play a role in the outcome of strabismus surgery. Few studies have evaluated "normal" position of EOM pulleys. We sought to assess intraoperatively medial rectus (MR) pulley location and variations in a prospective observational case series. Methods. A total of 194 consecutive patients with all types of strabismus aged 2 to 64 years had primary surgery on MR. We conducted 357 measures of the distance between scleral insertion of MR and anterior part of the pulley. We compared results to preoperative angle of deviation, age, refraction, and position of eyes under anesthesia. Results. Median location of the anterior part of the MR's pulley is 12.03 mm from scleral insertion, varying from 8 to 15 mm. There is a strong relation with refraction (p=0.005) and with preoperative angle of deviation (p=0.00002) and deviation under anesthesia (p=0.0001). The MR's pulley tends to be more anterior in hyperopic cases and esotropias, and posterior in myopias and exotropias. Conclusions. Our study shows physiologic variations of MR's pulley position with evidence of an adaptation to biometric parameters of the globe, represented by refraction state of the eyes in our study. Elsewhere, strabismus leads to an independent variation of MR's pulley location evaluated relatively to muscle's scleral insertion, but probably not if evaluated relatively to the orbit. We discuss consequences on ocular motility and surgery of these variations. They may explain some unexpected effects of strabismus surgery. Further studies are needed to ascertain their real impact.