Clinically referred children, diagnosed as having learning or attention disorders, with or without hyperactivity, were found to vary widely on a measure of nervous system sensitivity recommended by the Pavlovian investigator Vasilev. That is, the children were contrasted on their mean press and release reaction times (RIs) to four tones, ranging from soft (55 db) to very loud (100 db), with the expectation that some would be able to maintain a parallel separation of press and release RT gradients across all intensity levels (strength), whereas others would show convergence or overlap of the gradients at higher intensities (weakness). Contrary to expectation, girls did not have weaker or more sensitive nervous system than boys, although the girls rated themselves as less tolerant of intense stimuli. Significantly more of the children diagnosed as hyperactive had weaker nervous systems. The boys also participated in a blind crossover study contrasting placebo and methylphenidate effects; the prescribing physician, who was not informed of the child's nervous system classification, adjusted the dosage levels so that subjects with weaker nervous systems were titrated at higher dosage levels than those with stronger nervous systems. Gray (1964) suggested an explantation of this paradox, i.e., that a weak nervous system requires a more intense stimulus than the strong to reach the threshold of concentration (or focused attention), whereas for other thresholds the strong requires a more intense stimulus than the weak. Gray's theory was further supported by the finding that children typed as weak (unmedicated) did not show as great facilitation in RT with reward (moderate stimulus) as did those typed as strong.