The effects of pistachio by-products aqua extract (PBE) on growth performance and retention of dietary nitrogen (N) were investigated in finishing lambs fed diets containing 11.3% DM (low protein; LP) or 13.9% DM (high protein; HP) crude protein. Two groups (n = 10) of Baluchi ram lambs (4-5 months old, 26.25±3.30 kg initial BW) were allotted to LP and HP treatments. In each group, one half of the lambs receive d experimental diets mixed with 500 ml liquid fresh PBE per kg DM of diet. Thus, four experimental diets were tested: LP diet without PBE supplementation; LP-PEB, LP diet with PBE supplement ation; LP+PBE, HP diet without PBE supplementation; HP-PBE and HP diet with PBE supplementation; HP+PBE. Feed intake, average daily gain (ADG) and feed conversion ratio (FCR) were record ed for a 50 d experimental period . Plasma urea (PUN) and ruminal NH 3 -N concentrations were measured at 0, 2 and 4 h after morning feeding on the 35 th d of the experiment. Apparent total tract digestibility of DM, organic matter (OM) and N as well as urinary losses and retention of N were also determined for 5 days at the end of the experiment. Although there was a tendency for feed intake to be lower (P=0.102) in LP compared with HP fe d lambs, the dietary CP level had no effect on ADG and FCR. In contrast, PBE supplementation improved both growth rate (P=0.021) and FCR (P=0.104) of lambs. The urinary losses of N and also PUN and ruminal NH 3 -N concentrations at h 2 and 4 after feeding were decreased and the retention of N was increased by PBE supplementation of diets. However, these effects of PBE were on ly significant in the lambs fed HP diet. The concentrations of PUN (0 and 4 h after feeding) and ruminal NH 3 -N (4 h after feeding) were affected positively by dietary CP level. Although the lambs feeding on the LP diet retained a higher percentage of N intake or N digested, no difference in daily N retained was observed between LP and HP fed lambs. The experimental treatments had no effect on DM, OM and N a pparent digestibility. Our results indicated that the low concentration of PBE tannins have a protein-sparing potential depend on dietary CP density, thereby improving growth performance and N retention in finishing lambs

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