Spatially periodic turbulent fluid flow and heat transfer in a channel with slit rectangular ribs mounted on one wall have been numerically investigated. A Reynolds stress model with wall function and a modified wall-related pressure-strain model for reducing the overestimation of turbulent kinetic energy at impingement/reattachment region was employed in the two-dimensional simulation. QUICK discretization scheme and nonstaggered grid system were used. The parameters fixed were rib height-to-duct height ratio of 0.17, rib width-to-height ratio of 0.76, rib pitch-to-height ratio of 10, and Reynolds number based on channel hydraulic diameter and bulk mean velocity of 2 x 10(4), while the main parameter varied was the rib open-area ratio with values of beta = 0%, 10%, 22%, 32%, and 44%. Two critical ranges of beta and three characteristic flow regimes are identified, which is similar to the experimental results reported by previous researchers for the hole-type perforated ribs except a smaller critical beta for impermeability. The rationale for the difference in the critical beta between the slit-type and hole-type perforated ribs is addressed in terms of effective pore diameters. From the variations of the friction loss, convective mean velocity, turbulent kinetic energy, and negative pressure behind the perforated ribs with beta, that beta = 44% attains the best thermal performance (Nu) over bar (p)/(Nu(s)*) over bar under the constant pumping power condition is unraveled. A comparison of (Nu) over bar (p)/(Nu(s)*) over bar distributions between vertical-slit ribs S measured by other research groups and horizontal-slit ribs computed presently is also made.