Nerve conduction velocity (NCV) and efficiency of synaptic transmission are two possible biological mechanisms that may underpin intelligence. Direct assessments of NCV, without synaptic transmission, show few substantial or reliable correlations with cognitive abilities ["Intelligence" 16 (1992) 273]. We therefore assessed the latencies of reflexes involving one and many synapses to determine whether variations in speed of synaptic conduction are related to cognitive abilities and reaction times (RTs). NCV was estimated by measuring speed of nerve conduction in the patellar reflex arc and speed of finger withdrawal following electric shock. Speed and efficiency of neural transmission were indirectly measured via simple and choice RT tasks. Several hypothesized relationships were supported; however, the results tend to be inconsistent. The theoretical implications of the findings are discussed.