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This study examined the psychometric properties of an Argentine Version of the Morningness Composite (CS) and the Early/Late Preference (PS) Scales. During summer (long photoperiod in Argentina), 304 subjects (69.1% women; mean age=33.64 yrs, SD=14 yrs) completed the scales for the first time. In winter (short photoperiod), 100 of the same individuals (71% women; mean age=37.17 yrs, SD=14.63 yrs) were retested. The total scores ranged within the values reported by previous studies and were independent of gender. Older subjects showed higher morningness scores. The internal consistencies were good (CS=0.86, PS=0.82). Item 7 from the CS and items 7 and 9 from the PS showed low item-scale correlation. Factor analysis produced a three-factor solution for both scales. However, the inconsistency of the evening items suggests that the single-solution may be more acceptable. Test-retest correlations were satisfactory (CS=0.88, PS=0.78), but the two-related-sample test revealed significant differences between test and retest scores, suggesting relative temporal stabilities. Both scales presented similar and acceptable psychometric properties and good correlation, indicating construct validity.