In the present study, we have attempted to determine a detailed representation of the 5 alpha-Reductase (5AR) active site involving the elucidation of the transition state for the steroid delta 4 reduction reaction (the 'NADPH-substrate' complex), onto which steroidal and non-steroidal inhibitors were superimposed. We conclude that: (i) there is a requirement for groups to mimic the steroid substrate A-ring; (ii) the area about C(3), C(4), C(5) and C(6) of T appears to be sterically hindered, and; (iii) the area of the active site about the C(17) of the steroid substrate does not possess hydrogen bonding groups and is not restricted.