Put in to bat, Karnataka lost Robin Uthappa (11) and Abhishek Reddy (9) in the first hour to seamers Deepak Chahar and Tanveer-ul-Haq respectively as the visiting side were reduced to 51 for 2 in the Group A match at the Sawai Mansingh Stadium.
But Agarwal (66), playing his first game of the season, looked in fine touch during his third-wicket association of 85 with Samarth (65) before left-arm seamer Aniket Choudhary hit back.
Captain Vinay Kumar then made a patient 29 in Gopal's company to help Karnataka cross the 250-mark. Shreyas Gopal (40 batting) and David Mathias (16 batting) frustrated Rajasthan with a 38-run stand for the unbroken ninth-wicket stand when the stumps were drawn for the day.
For Rajasthan, Aniket Choudhary was the pick of the bowlers with figures of 3/72. Deepak Chahar and K Ajay Singh took two wickets each while Tanveer-ul-Haq got one wicket.
Karnataka 1st Innings: 270 for 8 in 90 overs (Mayank Agarwal 66, Ravikumar Samarth 65; Aniket Choudhary 3/72).