Despite numerous challenges and rising consumer demands, strategies for growing healthy animals and improving sustainability are being pursued, such as using natural, renewable sources to replace traditional feed materials. Thus, the aim of this study was to analyse the impact of macroalgal C. glomerata biomass on rabbit physiological features. The feeding trial (52-122 days) was carried out with twenty male Californian rabbits assigned to two dietary treatments (n=10 rabbits/treatment) and fed standard compound diet (SCD) and SCD + 4% C. glomerata (CG4). Macroalgal biomass was collected from the Lithuanian River Šventoji. At the end of the feeding trial, twelve rabbits (n=6 rabbits/treatment) were randomly selected and euthanized; samples were collected post-portem. A comprehensive analysis of rabbit organ development, histomorphometry, short-chain fatty acids (SCFA), and ammoniacal nitrogen (NH3-N) accumulation in intestinal contents was performed. The CG4 treatment had no impact on organ or intestinal development or the concentration of NH3-N in the duodenum (P>0.05). However, the crypt depth was significantly lower in the duodenum of CG4 compared to SCD, but 14.40 μm deeper in the ileum (P<0.05). Although CG4 had no impact on villus height, it had an impact on the villus/crypt ratio (V/C), which was higher in the duodenum in CG4 but lower in the ileum in SCD (P<0.05). After examining the gut contents from the duodenum, CG4 significantly reduced the lactic acid concentration, which was almost four times lower compared to SCD (P<0.05). Overall, C. glomerata in rabbit diets can improve the crypt depth in the ileum and therefore the absorption of nutrients that remain digested; nevertheless, it can reduce the crypt depth in the duodenum and thus the digestive area required to carry out the primary nutrient absorption. Moreover, such inclusion can reduce the concentration of lactic acid, whose decrease can affect weaker peristalsis and bowel movements
