A number of 18 male and 15 female weaned Barki lambs were used in two experiments to evaluate the comparative effect of dietary monensin or lasalocid on growth rate, rumen fermentation pattern, feed efficiency as well as age and weight at puberty. Male lambs (experiment 1) and female lambs (experiment 2) were randomly allotted to three equal groups: a control one which was fed on a non supplemented basal diet while, the other two ionophore treated groups were fed on the basal diet supplemented with 17 mg of monensin or lasalocid /head/ day, respectively. In both experiments, it was found that, total Volatile Fatty Acids (VFA), pH and butyrate cocentrations were not significantly affected by treatments. While, acetate(C2): propionate (C3) ratios were significantly (P<0.05) decreased by both additives, however, propionate was significantly (P<0.05) increased. Compared with the control group, monensin appeared to be more effective than lasalocid in increasing (P<0.05) propionate and reducing Cz:Cz ratio (P<0.05) and acetate levels. Compared to the control group, rumen ammonia level was lower (P<0.05) in lambs receiving monensin than those supplemented with lasalocid. In both experiments, Average Daily Gain(ADG) was higher (P<0.05) and feed efficiency was improved (p < 0.05) by the ionophore feeding. Ram lambs (exp. 1) fed on either monensin or lasalocid had a higher (P<0.05) net increase in scrotal circumference, besides that reached puberty earlier (p < 0.05) than those of the control group by about 50 and 37 days. The same was recorded by ewe lambs (experiment 2) as they reached puberty earlier the control group (P<0.05) by 35.4 and 13.4 days for monensin and lasolacid respectively, with the superiority of monensin (P<0.08). Progesterone concentration at puberty was significantly (p < 0.05) higher in monensin compared with lasalocid and control groups. On the other hand, monensin but not lasalocid decreased weight at puberty of both sexes than control.