This study investigated dietary supplementation of weaned lambs with Origanum vulgare essential oil and Capsicum oleoresin (chilli pepper) extract. Thirty-six eight-week-old male and female Menemen lambs were used in this study. Three dietary treatment groups consisted of T1, an unsupplemented control group; T2, a group supplemented with 300 mg/kg oregano essential oil, and T3, a group supplemented with 300 mg/kg Capsicum oleoresin. Feed and fresh water were available to the lambs ad libitum during the 56-day experiment. No significant effects of treatment were detected on growth rate, feed intake and feed conversion. In addition, serum urea, creatine, total protein, albumin, amylase, aspartate amino transferase (AST), alanine amino transferase (ALT), and gamma glutamyl transferase (GGT) levels did not differ among treatments on day 56 of the study. When oregano oil and capsicum extract were added to the feed, total volatile fatty acids (TVFA), acetate (AA), propionate (PA), butyrate (BA), isobutyrate (IBA), valerate (VA), and AA to PA ratio in the rumen were decreased significantly in comparison with the control group at two hours after feeding, with the effect of T2 being greater than that of T3. Female lambs had lower levels of TVFA than male lambs. Thus, although neither additive affected fattening performance and serum constituents of the lambs, both altered the rumen fatty acid profile.