Nitrogen (N) retention and wool growth rate (WGR) responses to various amino acids and derivatives as well as protein sources were determined with six ruminally and six abomasally fistulated Merino wethers in two balanced cross-over design experiments. The six treatments of the first trial consisted of a control (RC), ruminally infused: methionine (RM), maleyl methionine (RMM) and 2-methyl-maleyl methionine (RMMM), as well as other essential amino acids supplied by a low (RFM: fish meal) and high degradable dietary protein source (RSOM: sunflower- oil cake meal), respectively. The latter two treatments (AFM and ASOM, respectively) and a control (AC) were included in the second trial as well as abomasally infusion of methionine (AM) and/or lysine (AL or AML). Six roughage-based diets were compiled to be iso-nutritious and complementary to the different treatments and were fed at a maintenance level. Live weight was not affected (P > 0.10) by any treatment whereas the apparent DM digestibility was enhanced (P < 0.05) with the inclusion of fish meal in the diets (RFM: 56.7%; AFM: 57.5%). The percentage N retained (15.6 and 35.8% vs. 26.1%) decreased (P = 0.03) by ruminally infused methionine (RM) and increased (P = 0.04) by inclusion of fish meal (RFM). Abomasally infusion of methionine (AM) resulted in an increased (P = 0.02) percentage N retained (39.0%) but the infusion of lysine (AL) or both (AML) gave no response (26.5 and 36.6% vs. 30.1%). WGR was enhanced by 0.21 ± 0.05 mg/cm2/day (P = 0.0002) and 0.27 ± 0.05 mg/ cm2/day (P = 0.0001) by ruminally infusion of derivatives (RMM and RMMM) and the inclusion of natural protein sources (RSOM and RFM), respectively. Abomasally infusion of methionine (AM) and a mixture of amino acids (AML) as well as diets containing natural protein sources (ASOM and AFM) increased (P < 0.0004) WGR with 31, 35, 19 and 38%, respectively, whereas lysine infusion (AL) had no effect. MMM (2-methyl-maleyl methionine) and fish meal show great potential as a source of bypass methionine, especially to increase wool growth (30% and 36-38%), whereas MM (maleyl methionine) (12%) and sunflower-oilcake meal (17-19%) appears to be about half as efficient.