Objective To investigate the effect of lens-induced defocus on emmetropization in guinea pigs that lack accommodation.Methods One hundred twenty pigmented guinea pigs with complete cycloplegia (7 days after birth,except for group Ⅰ) were randomly assigned to 6 groups.At the start of the study,20 guinea pigs for each group.In the final data analyses,15 for all groups except for group 1 with 20.The groups were divided as follows:Ⅰ:no lens,with physiological saline eye drops (qd,OU,control group); Ⅱ:no lens,with 1％ atropine eye drops (qd,OU,which caused cycloplegia).Ⅲ/ Ⅳ/ Ⅴ/ Ⅵ:-2/-4/-6/-8 D lenses with 1％ atropine eye drops,OU.Refraction (mean spherical equivalent [MSE]),axial length and corneal curvature of the right eyes were measured on day 1 and day 21.Another 5 pigmented guinea pigs were used to measure the effect of cycloplegia on pupil size (OS:1％ atropine eye drops,qd.OD:physiological saline eye drops,qd).Statistical analysis was performed with linear correlation,linear regression,an independent samples t test and ANOVA.Results There were no significant differences in all data between the two non-de focus groups.The development of corneal curvature was similar among imposed defocus groups.All defocus groups showed relative myopic refractive errors.There were significant differences between the imposed defocus groups and non-defocus group Ⅱ.With the increase in relative myopic refractive errors,there was a further elongation in axial length.Between the defocus groups,Ⅲ and Ⅳ were significantly different from Ⅵ for both refractive error and axial length.Conclusion Axial elongation and relative myopia are observed in lens-induced developing eyes that lack accommodation.Both axial elongation and myopia increase with an increase in imposed defocus.Thus,at least in guinea pigs,developing eyes are able to grow to compensate for various degrees of imposed defocus even when accommodation is lacking.