1. Most studies of cortical neurons have focused on the spatial structure of receptive fields. For a more complete functional description of these neurons, it is necessary to consider receptive-field structure in the joint domain of space and time. We have studied the spatiotemporal receptive-field structure of 233 simple cells recorded from the striate cortex of adult cats and kittens at 4 and 8 wk postnatal. The dual goal of this study is to provide a detailed quantitative description of spatiotemporal receptive-field structure and to compare the developmental time courses of spatial and temporal response properties. 2. Spatiotemporal receptive-field profiles have been measured with the use of a reverse correlation method, in which we compute the cross-correlation between a neuron's response and a random sequence of small, briefly presented bright and dark stimuli. The receptive-field profiles of some simple cells are space-time separable, meaning that spatial and temporal response characteristics can be dissociated. Other cells have receptive-field profiles that are space-time inseparable. In these cases, a particular spatial location cannot be designated, unambiguously, as belonging to either an on or off subregion. However, separate on and off subregions may be clearly distinguished in the joint space-time domain. These subregions are generally tilted along an oblique axis. 3. Our observations show that spatial and temporal aspects of receptive-field structure mature with clearly different time courses. By 4 wk postnatal, the spatial symmetry and periodicity of simple-cell receptive fields have reached maturity. The spatial extent (or size) of these receptive fields is adult-like by 8 wk postnatal. In contrast, the response latency and time duration of spatiotemporal receptive fields do not mature until well beyond 8 wk postnatal. 4. By applying Fourier analysis to spatiotemporal receptive-field profiles, we have examined the postnatal development of spatial and temporal selectivity in the frequency domain. By 8 wk postnatal, spatial frequency tuning has clearly reached maturity. On the contrary, temporal frequency selectivity remains markedly immature at 8 wk. We have also examined the joint distribution of optimal spatial and temporal frequencies. From 4 wk postnatal until 8 wk postnatal, the range of optimal spatial frequencies increases substantially, whereas the range of optimal temporal frequencies remains largely unchanged. From 8 wk postnatal until adulthood, there is a large increase in optimal temporal frequencies for cells tuned to low spatial frequencies. For cells tuned to high spatial frequencies, the distribution of optimal temporal frequencies does not change much beyond 8 wk postnatal.(ABSTRACT TRUNCATED AT 400 WORDS)