The development of statistical methodologies enabling quantitative study of natural images has brought about increased interest in measuring neural activity in response to natural stimuli. What is known about the response of primary visual cortex to image parameters such as contrast or spatial frequency has come from studies completed with stimuli such as checkerboards or gratings, but the correspondence between these functions and the response of the visual system to natural inputs is not certain. We have developed a parameterization of natural images that characterizes the degree of "naturalness" of an image based on a small number of parameters derived primarily from the spatial frequency spectrum of the image (contrast energy, and log amplitude spectrum slope, and phase coherence). By presenting rapid sequences of images, drawn at random for each frame and varying between natural and unnatural in a controlled fashion, we have used high field BOLD fMRI to quantify the response of primary visual cortex to natural images. We believe that the technique of rapid presentation substantially reduces temporal blurring by the hemodynamic response function, a known limitation of BOLD fMRI measurements, and thus response can be measured rapidly. As an initial test of the viability of this method, a sequence of images was generated in which image r.m.s. contrast was modulated from frame to frame. The images were presented in rapid succession as the contrast energy was smoothly varied. The modulation of the positive BOLD response was correlated with modulation of contrast power. The measured modulation of the signal is in good agreement with previous fMRI studies using sine wave gratings to measure BOLD signal tuning to image contrast.