This study examined the effects of floor space and feeder space on the feeding behaviour, growth and stress physiology of lambs, at different times, over a 6-week period in a finishing feedlot. A total of 640 lambs in four time replicates (160 lambs per replicate) were studied in four 40-lamb intensive finishing systems (feedlots) for 6 weeks over a 16-month period. The lambs were Merino wethers with live weight at the commencement of the experiment of 28.3 (s.d. = 2.4) kg. The effects of two factors, pen floor space (2 and 5 m2/lamb) and feed trough space (4 and 10 cm/lamb) were examined in a factorial arrangement. Measurements taken on individual lambs during the experimental period included: feeding behaviour and displacements at the feeder in the feedlot during weeks 1, 2 and 6, lying behaviour during week 3, weekly live weights and cortisol concentrations in weeks 1 and 6. Weekly feed intake for each group of lambs was also record. Trough space of 4 cm compared to 10 cm per lamb reduced average lamb feed intake in all weeks of the 6-week period (a reduction of 1.2 (s.e. = 0.53) kg/hd in week 1, 1.3 (s.e. = 0.62) kg/hd in week 2, weekly average of 0.9 (s.e. = 0.17) kg/hd over weeks 3–6; P = 0.05, P = 0.06 and P = 0.0004, respectively). However, weight gain only differed significantly during the first week in the feedlot (average of 0.2 vs 1.1 (s.e.d. = 0.35) kg for 4 cm vs 10 cm/lamb of provided trough space, P = 0.03). At all observed times within the 6-week period, lambs with the reduced feed trough space spent less total time feeding (P < 0.05) with less feeding bouts (P < 0.05). There was no effect of feed trough space on lying time or cortisol concentrations. Floor space did not affect feeding and lying behaviour, feed intake, weight gain or cortisol concentrations. In conclusion, increasing feed trough space from 4 to 10 cm/lamb increased feed intake and live weight gain through an increase in the number of feeding bouts and the total time feeding. This indicates that the trough space recommendations in the Australian Model Code of Practice (sheep) of 2 cm/lamb are too low for maximum lamb feed intake and live weight. No effects of increasing floor space from 2 to 5 m2/lamb were found, however the growth rates in general were low suggesting that the environment in the feedlot may have been sub-optimal.