We analyse the temperature and magnetic field dependence of the power-law n-values characterizing the electric field-current density (E-J) curves of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconducting samples. This is done by means of magnetic relaxation measurements, using the normalized relaxation rate of irreversible magnetization M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">irr</sub> , S = -dlnM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">irr</sub> /dlnt, which directly relates to the E-J curve E infin J <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sup> with the n-exponent approximately equal to 1 + 1/S. The study has been done at temperatures and fields ranging from (T = 5 K, B = 1 T) and up to (T, B) near the irreversibility line. Different samples such as undoped and SiC-doped bulk samples and wires have been analysed. We have observed a strong decrease in the n-values when increasing temperature and magnetic field with some differences among the analysed samples. We found correlation between critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) and n-values. The J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (T, B) and n(T, B) dependences show striking similarities and the high n-values (n > 30) are always related to the high critical current densities (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> > 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> A/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ).