The objective of this paper are the proof issues of the exceptio doli in the formular process. The author examines different texts from Roman lawyers regarding not only the proof of the exceptiones, but of the dolus as well, turning back to the problem of onus probandi in the classical process. One of the premises of this work is that the Exceptio doli has a bigger complexity than other exceptiones. These issues found a safer treatment when created the exceptio non numeratae pecuniae. The hypothesis is that this Exceptio should be seen as an alternative to the Exceptio doli every time the defendant has no chance to prove the facts that constitute the plaintiff's Dolus (a positive fact). And this situation would be related to the fact that the reversal of burden of proof in case of negative facts was sufficiently known in the classic process and therefore the rescript of C. 4,30,3 was unnecessary for that goal.