Abstract <p>This study aimed to investigate the effects of oral administration of bovine rumen fluid on certain physiological parameters of <i>Oncorhynchus mykiss</i>. Sixty rainbow trout, each weighing approximately 125&#xa0;g, were randomly selected and divided into four groups. They were fed daily with specific amounts of bovine rumen fluid: Group (T1) received 1.6 mL/kg body weight, Group (T2) received 3.2 mL/kg body weight, and Group (T3) received 3.2 mL/kg body weight of boiled rumen fluid. The control group didn’t receive bovine rumen fluid. After two months, all fish were collected, and their feed conversion ratio was measured. Additionally, biochemical and microbiological analyses were performed. A statistically significant difference was observed between the control and treatment groups in terms of total intestinal microbiota count and blood glucose levels (<i>P</i> &lt; 0.05). However, no statistically significant difference was found in uric acid levels between the control and treatment groups (<i>P</i> &gt; 0.05). Furthermore, significant differences were detected in blood total protein levels and liver enzymes, including AST and ALT, between the control and treatment groups (<i>P</i> &lt; 0.05). Regarding FCR, significant differences were noted between the control and treatment groups, except for treatment group 2 (<i>P</i> &lt; 0.05). The positive effects of bovine rumen fluid diminished. The oral administration of bovine rumen fluid positively influences growth performance and improves certain physiological parameters in rainbow trout. Therefore, this cost-effective and readily available resource can be used at appropriate doses to enhance the efficiency of rainbow trout farming.</p>