Recent clinical and preclinical research has suggested that cannabidiol (CBD) and Δ9-tetrahydrocannabinol (Δ9-THC) have interactive effects on measures of cognition; however, the nature of these interactions is not yet fully characterized. To address this, we investigated the effects of Δ9-THC and CBD independently and in combination with proposed therapeutic dose ratios of 1:1 and 1:3 Δ9-THC:CBD in adult rhesus monkeys (n = 6) performing a stop signal task (SST). Additionally, the development of tolerance to the effects of Δ9-THC on SST performance was evaluated by determining the effects of acutely administered Δ9-THC (0.1-3.2 mg/kg), during a 24-day chronic Δ9-THC treatment period with Δ9-THC alone or in combination with CBD. Results indicate that Δ9-THC (0.032-0.32 mg/kg) dose-dependently decreased go success but did not alter go reaction time (RT) or stop signal RT (SSRT); CBD (0.1-1.0 mg/kg) was without effect on all measures and, when coadministered in a 1:1 dose ratio, did not exacerbate or attenuate the effects of Δ9-THC. When coadministered in a 1:3 dose ratio, CBD (1.0 mg/kg) attenuated the disruptive effects of 0.32 mg/kg Δ9-THC but did not alter the effects of other Δ9-THC doses. Increases in ED50 values for the effects of Δ9-THC on SST performance were apparent during chronic Δ9-THC treatment, with little evidence for modification of changes in sensitivity by CBD. These results indicate that CBD, when combined with Δ9-THC in clinically available dose ratios, does not exacerbate and, under restricted conditions may even attenuate, Δ9-THC's behavioral effects. (PsycINFO Database Record