There are now extensive behavioral and neuropsychological evidence to indicate that semantic information of a word can be activated without conscious awareness. However, semantic activation alone may not be sufficient for observing semantic priming effects in masked lexical decision task. In the following study, two tasks were used: lexical decision and semantic categorization. Conscious awareness of the prime was systematically manipulated by varying the duration of the prime and by varying the placement of the mask in the prime-target presentation sequence. Priming effects were observed in the semantic categorization task at prime durations of 42 milliseconds but no semantic priming was observed for the same prime duration in the lexical decision task. However, semantic priming effects began to emerge in lexical decision at the longer prime durations (55 & 69 ms) and under the least effective prime-mask presentation sequences. It is proposed that semantic activation alone is not sufficient for semantic priming effects in the lexical decision task but that central executive involvement is necessary, if only at the lowest level, for facilitatory effects to be observed. Furthermore, no such central executive involvement appears to be required for the semantic categorization task. The priming effects obtained in this task is interpreted in terms of a "decision priming" effect.