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The status of blocking in human eyelid conditioning was examined in a series of four experiments modelled on rabbit nictitating membrane (NMR) procedures to ensure comparability with the animal literature. The first three employed Tones and Lights as CSs and a preliminary experiment established equivalent salience of these stimuli. The fourth employed all light conditioned stimuli (CSs). Each of the experiments balanced order of presentation of blocked and non-blocked stimuli in extinction; each yielded reliable evidence of blocking tested in extinction. However, the blocking effects were attenuated or abolished in those groups receiving the to-be-blocked stimulus on the first trial in extinction. The results show blocking to be a relatively weak, easily disrupted phenomenon compared with rabbit studies. They suggest that rapid re-appraisal of stimulus significance plays a more important role in human subjects than has been observed in animal studies.