Abstract. Nanotubical structures are obtained by wrapping a hexagonal grid, and then possibly closing the tube with caps. We show that the size of a cap of a closed (k,l)-nanotube depends only on k and l. Consequently, we show that the asymptotic values of Wiener, Schultz and Gutman indices for nanotubical graphs (open or closed or generalized) are n3/(6k+6l), 3n3/(2k+2l) and n3/(k+l), respectively.