Projected future CO₂ levels in the ocean can alter the behaviour of marine animals. Disrupted functioning of the γ-aminobutyric acid type A (GABA_A) receptor is suggested to underlie CO₂-induced behavioural changes in fish, however, the mechanisms underlying behavioural changes of marine invertebrates at elevated CO₂ levels are not well understood. We exposed two-toned pygmy squid Idiosepius pygmaeus to ambient (~450 µatm) or elevated (~1,000 µatm) CO₂ levels for seven days. Squid were treated with sham, gabazine (GABA_A receptor antagonist) or picrotoxin (chloride (Cl^–) channel blocker) immediately before measurement of conspecific-directed behaviours and activity levels upon mirror exposure. If disrupted function of GABA_A-like and/or other Cl^– channel receptors underlies the behavioural changes, we predicted that gabazine and picrotoxin would attenuate the behavioural changes at elevated CO₂. Elevated CO₂ increased squid activity levels and altered some, but had no meaningful effect on other, conspecific-directed behaviours. Gabazine and picrotoxin attenuated some of the behavioural changes at elevated CO₂, indicating altered GABA_A-like and Cl^– channel receptor functioning may underlie these behavioural changes. However, gabazine and picrotoxin had the same effect at both CO₂ levels on other behavioural traits, suggesting altered function of GABA_A-like and Cl^– channel receptors was not responsible for other behavioural changes at elevated CO₂. Our results suggest multiple mechanisms may be involved, which could explain variability in the effects of CO₂ and drug treatment across behaviours.