The normal post-natal development of visual cortical functions was studied by recording extracellularly from 612 single neurones in the striate and parastriate cortex of anaesthetized and paralysed kittens, ranging in age from 6 to 24 days. Analyses have been made of laminar differences in the developmental trends of receptive field properties such as orientation specificity and spatial organization of 'on' and 'off' zones. At the beginning of the second post-natal week the majority of neurones (76%) only respond to light 'off' (unimodal 'off' neurones). Only later does the frequency of occurrence of unimodal 'on' neurones and of bimodal or multimodal neurones (with spatially segregated 'on' and 'off' zones arranged side by side) increase so that, by the middle of the fourth week, about equal numbers of these three receptive field types are found. The proportion of 'on-off' neurones (with spatially coincident 'on' and 'off' zones) remains low (between 9% and 12%) during the early post-natal period. In layers 4 and 6 of areas 17 and 18 the frequency of occurrence of visual neurones is quite normal even in the youngest kittens, whereas the probability of recording neurones in layers 2/3 and 5 in kittens less than 14 days old is remarkably low and only gradually improves up to the middle of the fourth week. A very rudimentary order in the spatial arrangement of orientation-specific neurones and ocular dominance distribution is observed even in very young kittens. This order improves rapidly and reaches adult levels during the fourth post-natal week. In visually inexperienced kittens, on average 11% of all responsive neurones are selective for the orientation of elongated visual stimuli, and 58% are biased. The proportion of orientation-selective cells begins to increase rapidly about two days after lid opening, and proportions of orientation-selective cells similar to that in the adult are reached by the end of the fourth post-natal week. Orientation-selective neurones in kittens less than 10 days old are only found in layers 4 and 6 and the lower part of layer 3. In layers 2/3 and 5 they are first seen in larger proportions by the beginning of the third post-natal week. Our results show that, during the first post-natal month, the time course of the functional development of visual cortical neurones depends on receptive field type and on intracortical location.(ABSTRACT TRUNCATED AT 400 WORDS)