Introduction: Extraocular muscle tenotomy has been reported to damp congenital nystagmus in an achiasmatic sheepdog. We performed extraocular muscle tenotomy to evaluate its effects on congenital nystagmus in primates. Methods: Magnetic search coil eye movement recordings were used to document the presence of horizontal congenital nystagmus in 2 adult macaque monkeys that also had naturally occurring infantile strabismus. Extraocular muscle tenotomy was performed by operating on all 4 horizontal recti, surgically detaching the muscles from the globe and suturing them back to their original insertions without resection or recession. Eye movement recordings were repeated 4 months after the procedure, comparing the waveform, amplitude, retinal slip velocity, and intensity (frequency x amplitude) of the nystagmus before and after tenotomy. Visual acuity was also measured before and after surgery in 1 animal. Results: Preoperatively, a disconjugate, pendular nystagmus was evident in 1 monkey, and the other had a conjugate pendular-jerk nystagmus damped by convergence. After tenotomy, nystagmus mean amplitude decreased 18% to 52% in 1 monkey but increased 14% in the other (t test, P < .002). Retinal slip velocity and nystagmus intensity increased in both monkeys. After tenotomy, mean velocity increased 22% to 218%, while mean intensity increased 40% to 208% (t test, P < .002). Visual acuity measured after tenotomy decreased an average of 20% (∼ 2.0 cycles per degree) in each eye. Tenotomy had no noteworthy effects on eye alignment or other aspects of visual behavior other than the congenital nystagmus. Conclusion: Nystagmus velocity and intensity increased after extraocular muscle tenotomy in 2 monkeys. Further studies are required to establish the clinical value of this procedure as a treatment for various subtypes of congenital nystagmus in humans.