Interpublic had estimated its earnings to be between 40-45 cents (29p-32p) per share in the second quarter. It now believes its earnings per share will be in the 30-35 cents (21p-25p) range.
True North said it would miss analyst estimates of 49 cents (35p) a share and expects to earn 37-42 cents (26p-30p) a share.
David Bell, chairman and chief executive of True North, said, "These results are indicative of the general economic slowdown and not a reflection on the strength of True North's brands."
True North shareholders are due to vote on the merger with Interpublic on June 19. Interpublic's chairman and chief executive John Dooner Jr said that details of the initiatives to combine the companies and create a "New Interpublic" would be released in mid-July.
Integrating the two companies will lead to "significant cash and non-cash charges", estimated at more than $300m (£213.9m) in the second and third quarters of 2001.