The conditions for high level of transcription of structural genes in Lac operon are
I. absence of Glucose                                     II. presence of Lactose 
III. active Repressor                                        IV. inactive repressor
 

In the absence or very low levels of glucose as energy source and in the presence of lactose the structural genes of lac operon are transcribed. If glucose is present in the medium, there is no need of using lactose as energy source. In the absence of glucose and presence of lactose, lactose binds to the repressor protein and does not allow the repressor to bind to operator and converts it into inactive repressor. This promotes binding of RNA polymerase to the promoter of structural genes and transcription takes place and mRNA of structural genes is formed which is translated to form the gene products of z, y and a, as β-galactosidase, permease and transacetylase.