Abstract Background TDCS modulates cortical excitability in a polarity-specific way. When used in combination with a behavioural task, it can also alter performance. Previously, tDCS modulated the performance of older adults on a complex speech motor learning task, which involved repetition of tongue twisters . Objective We aimed to replicate this finding in healthy young participants and to extend it by measuring tDCS-induced changes in motor excitability using transcranial magnetic stimulation and motor-evoked potentials elicited in the lips. Method In a double-blind randomized sham-controlled study, three groups of 20 participants received: 1) anodal tDCS to the left IFG/LipM1 and cathodal tDCS to the right hemisphere homologue; or 2) cathodal tDCS over the left and anodal over the right; or 3) sham stimulation. Participants heard and repeated tongue twisters and matched simple sentences before, during and 10 minutes after the stimulation. Motor excitability was measured before and immediately after the tDCS. Results The improvement in performance of tongue twister repetition from baseline to after stimulation was significantly greater than for the simple sentences but did not differ among the three groups. Motor excitability significantly decreased to a small but similar extent across the three groups. Conclusions TDCS did not modulate performance on a complex articulation task in healthy young adults. TDCS applied concurrently with task learning also failed to modulate motor excitability in expected ways. TDCS may be most effective in brains where brain function is sub-optimal due to age-related declines or pathology.