Atriplex nummularia L. (atriplex) foliage and urea-treated straw have been used as nitrogen supplements for Barbarine lambs given Opuntia ficus-indica f. inermis (cactus) and their effects on feed intake, diet apparent digestibility, nitrogen balance and daily gain have been evaluated. Twenty-four lambs (mean live weight 19·9±2·79 kg) were randomly allocated to one of four experimental groups, groups 1–3 received freshly cut pads of cactus ad libitum. Group 1 was supplemented with 600 g untreated barley straw (C+US), while group 2 received 600 g urea-treated straw (C+UTS). Group 3 received 600 g untreated straw and 400 g atriplex (C+A). The amount of atriplex was calculated so that groups 2 and 3 receive iso-nitrogenous diets (i.e. C+UTS and C+A). Group 4 (US+BW) received a diet commonly offered to sheep by Tunisian smallholders, untreated straw ad libitum and 400 g of ground barley grain and wheat bran mixture (50[ratio ]50). A growth trial (60 days) followed by a digestibility trial (8 days) were performed in 1999. Irrespective of dietary treatment, cactus intake was high, averaging 500 g dry matter (DM)/day. It was not affected (P>0·05) with N supplementation, as urea-treated straw or atriplex. Sheep fed cactus-containing diets drank considerably less (P<0·001) water than those on the common diet (US+BW). Nitrogen provision improved significantly apparent digestibility of cactus-based diets. Urea treatment of straw increased content of digestible dry matter, organic matter, crude protein and cell wall (NDF) by 100, 100, 120 and 290 g/kg diet, respectively. There was a further increase when atriplex was provided instead of urea-treated straw. Nitrogen balance was positive across all dietary treatments but was improved when the nitrogen content of cactus-based diets was increased. Sheep fed on cactus and supplemented with atriplex retained the same amount of N as those fed the common diet (US+BW). The efficiency of microbial N production, estimated by the urinary excretion of allantoin, was improved (P<0·05) with N supplementation of cactus-diets (5·7, 12·4 and 11·4 mg/kg digestible OM intake for diets C+US, C+UTS, C+A, respectively). The diet C+US covered maintenance requirements of the sheep (7·7 g of daily gain). Nitrogen supplementation of cactus-based diets raised the average daily gain of lambs. Indeed, sheep given C+UTS or C+A diets grew at a rate of 19·9 or 31·4 g/day, respectively. It is noteworthy to mention that C+A and US+BW diets had similar effect (P>0·05) on sheep growth (31·4 and 39·7 g/day, respectively). It is concluded that cactus may be considered as an emergency feed and water source for sheep in arid and semi-arid zones. Nitrogen supplementation of cactus-based diets with urea-treated straw or atriplex foliage improved the feeding value of these diets and consequently sheep growth. A cactus-based diet, supplemented with atriplex, promoted similar growth in sheep as the commonly used diet (US+BW).