What did Calvin think about Knox's "Monstrous Regiment"? Scottish Protestant reformer John Knox (1514-1572) is the author of the work of vile misogyny, The first blast of the trumpet against the monstruous regiment of women (1558). In the book, Knox rails against the idea of women having authority over men, particularly in the case of queens. He calls them "weak", "sick", "impotent", "foolish", "mad", "frenetic", "impatient", "feeble", "inconstant", "cruel", "corrupted", and so forth.  Mary I of England was "that cursed Jezebel" and "horrible monster", "bloody tyrant"; Mary of Guise was "that crafty dame". He called for the people to rise up to murder both of them. He also hates "the odious nation of Spaniards", Catholics ("pestilent and detestable"), and frankly quite a lot of other things.

In a comment on an answer to an earlier question, I said that John Calvin (Knox's mentor) disapproved of what Knox had written. On reflection I realize that I don't actually have any evidence for that. I was misremembering a different comment by Calvin where he said "moderate your rigor" to Knox, but that was in the context of tolerating variations in the conduct of public worship (Knox absolutely hated any deviation from his ideas).

What did Calvin actually think about the Monstrous regiment?  - both in terms of the political difficulty it created, and in terms of the actual content (that is, did he agree or disagree with the basic sentiments, independently of the trouble the book caused?).
It seems that your question is a settled part of Church History, judging from David Calhoun's lecture "Blowing the Trumpet: John Knox and the Scottish Reformation" (quoting from the transcript; see the audio recording and the study guide):


  Knox said women should not rule over men. That was a kind of call for revolution. People could read between the lines and realize that what John Knox really wanted was for men in the two countries to rise up and overthrow the women rulers and bring in Protestant rulers. John Knox wrote the book in Geneva. It was an embarrassment to John Calvin. Calvin explained that when Knox came to him with the idea of writing such a book, Calvin strongly discouraged him from doing it. Calvin pointed to Deborah and Huldah, women in the Old Testament who were legitimate rulers. Calvin was also concerned about Knox's approach, because Calvin was very conservative in his view of overthrowing kingdoms. He told the Huguenots in France that it was better to suffer than to create anarchy and revolution. Calvin did not open much of a door for any kind of revolution. John Knox did open such a door in his First Blast of the Trumpet for revolution in Scotland and elsewhere.