It is largely accepted that only a few stimuli will be selected and attentively analyzed in a difficult visual search task, but the efficiency of the selection process remains unclear. The present study shows that the processes of selection and analysis are affected by different experimental factors, which contribute additively to the RT. The RT increases as a function of the target-distractor similarity and, as a function of the presence of the background stimuli increases the RT in 30ms, independent of target-distractor similarity. In the target absent trials, the effect of the background stimuli is inversely related to target-distractor similarity. The increase in similarity between target and distractor can lead to and increase in the ratio of guessing, compensating or eliminating the increase in RT caused by the presence of the background stimuli. These results are compatible with a model which the relevant stimuli are selected pre-attentively and then, submitted to and attentive serial analysis.