U.S. shares of Actelion and Sanofi were down 9 percent and 2.5 percent, respectively, on Wednesday.
It's unclear what price Sanofi might pay or what structure is envisioned, but the deal could value Actelion as much as $30 billion, according to The Wall Street Journal, which first reported on the talks and cited people familiar with the matter.
On Tuesday, Johnson & Johnson ended its discussions with Actelion regarding a possible transaction. Actelion told J&J it was confident it could attract a higher offer than the 250 Swiss francs per share the U.S. company had offered, Reuters reported.
—CNBC's David Faber and Reuters contributed reporting to this story.