<p> Feeding reduced protein diets may influence the microbial fermentation in the ceca due to the differences in the diet composition and nutrient digestibility compared to the standard protein diets. Two experiments were conducted to investigate the effects of L-arginine (Arg) and L-citrulline (Cit) supplementations in reduced protein diets on cecal fermentation products including ammonia and fatty acids in broilers raised under standard, cyclic warm temperature (WT), and necrotic enteritis (NE) challenge conditions. Sub-objectives were to examine preserving methods for cecal digesta (snap freezing using liquid nitrogen <i>vs</i> ice) and analytical methods (enzymatic assay <i>vs</i> Berthelot method) to determine cecal ammonia. In experiment 1, Ross 308 cockerels (<i>n</i> = 720) were randomly allocated to four dietary treatments with 12 replicates of 15 birds per pen from d 7 to 21. The treatments were: a standard protein diet (SP), a reduced protein diet with 2.5 percentage points lower crude protein (CP) and deficient in Arg (RP), and RP supplemented with 0.28% Arg (RP-Arg) or 0.28% Cit (RP-Cit). A factorial arrangement of treatments was applied during the finisher phase (21 to 35 d). Factors were: diet (four diets above); and temperature, NT (24°C) or cyclic WT (33°C ± 1°C for 6 h per day) using 6 replicate pens per diet per temperature. In experiment 2, Ross 308 cockerels (<i>n</i> = 720) were randomly assigned to six experimental treatments with eight replicates of 15 birds per pen. The treatments were: a standard protein without NE challenge (SP-); with NE challenge (SP+); reduced protein (1 to 2 percentage points lower CP) without NE challenge (RP-); with NE challenge (RP+); RP+ plus added Arg (103% of Ross 308 requirement, RPA+) and RPC+ where supplemental Arg-in RPA+ was replaced with Cit. The results of experiment 1 showed that birds fed the Arg-deficient RP diet tended to have lower total cecal ammonia levels (<i>P</i> = 0.054) on d 21, and had higher plasma ammonia levels on d 35 (<i>P</i> < 0.05) compared to those fed the SP. Supplementation of Arg-or Cit in the Arg-deficient RP diet reduced plasma ammonia level (<i>P</i> < 0.05) but did not affect total cecal ammonia level compared to the Arg-deficient RP treatment on d 35. Necrotic enteritis challenge decreased total cecal ammonia level (<i>P</i> < 0.01, experiment 1) while cyclic WT did not affect this parameter (experiment 2). Birds exposed to cyclic WT had a lower cecal formic acid level compared to those reared in NT on d 35 (<i>P</i> < 0.05). Preserving samples on ice or by snap freezing with liquid nitrogen during sample collection did not affect recovery of cecal ammonia. Determination of free ammonia level using an enzymatic assay using the Thermo Indiko Plus technique can be used to replace the Berthelot method as a quick assessment tool to evaluate cecal ammonia level in birds.</p>