The temporal course of startle reflex modulation and autonomic response patterns to fear-relevant and fear-irrelevant pictures in subjects with high and low levels of animal fear was investigated. Thirty-eight high-fear and 48 low-fear volunteers viewed photos of snakes and spiders and pictures of neutral and pleasant content. The slides were presented for 6 s or for only 150 ms, depending on the group. Acoustic startle probes were presented at five different times after slide onset. Relative potentiation of the startle responses started 300 ms after onset of snake/spider pictures in fearful subjects. This fear-potentiated startle effect was maintained for the later probe times and was identical in the 150-ms condition. Fear-relevant pictures also prompted a sympathetically dominated autonomic response profile in fearful persons. These data support the idea that fear can be activated very rapidly, requiring only minimal stimulus input.