Data on 436 Mehraban fat-tailed ewe lambs collected during 1985 to 1988 were analysed. A 4-year study of factors affecting age and body weight at puberty in Mehraban ewe lambs showed that the year of birth did not affect age at puberty but body weight at puberty. Sires significantly influenced age and body weight at puberty with body weight being more important than age. Sire-group correlation of age at puberty between years was low while that of body weight was relatively high. Date of birth within the year had a significant effect on age and body weight at puberty. Ewe lambs born late in the lambing season were lighter and younger at puberty. High pre-weaning growth rate and heavy weaning weight were associated with early puberty and heavy body weight at puberty. Age and body weight at puberty averaged 244.7 (s.d. 13.9) days and 44.1 (s.d. 2.4) kg respectively. Heritability estimates for age and body weight at puberty were 0.14 and 0.37 respectively.