Metamorphoses, 3.39

Can anyone please help me to understand the word ratae in line 341 in Book 3 of Metamorphoses?

Prima fide vocisque ratae temptamina sumpsit

Is this the past participle of reor, used as a noun? If so, what would be the best translation of it?

It’s properly the past participle of reor, yes, but here as often it’s used as a simple adjective meaning valid or authoritative, or just true, as here of Tiresias’ prophetic “voice.”
fide is also genitive (archaic but metrically convenient for fidei), so fide vocisque make a pair. Liriope was the first to put it the test (temptamina sumpsit).

Ovid’s sumptuous tale of Narcissus and Echo is a perennial favorite. Enjoy!

Thank you for this wonderfully clear explanation! Carl