Conditioned blocking (CB) measures the transient suppression of learning that a new stimulus, added during learning, has the same consequences as the conditioned stimulus already present. Normal CB increases between the age of 8 and 20 years (Oades, R.D., Roepcke, B. and Schepker, R., A test of conditioned blocking and its development in childhood and adolescence: relationship to personality and monoamine metabolism, Dev. Neuropsychol., 12 (1996) 207–230). In the present study CB development is compared between healthy children (CN), children with attention deficit (ADHD) and those with complex tics or Tourette's syndrome (TS) with mean ages of 10–11 years. All children needed fewer learning trials with increasing age: the ADHD group showed a slight impairment. Only controls improved CB with increasing age. A trend for worse CB in the TS than the other groups was significant for those over 11 years. While ADHD children over 11 years showed less CB than controls, younger ADHD children showed more. A correlational analysis of the status of monoamine metabolism in 24 h urine samples showed a positive relationship for CB with dopamine metabolism in controls and TS children, but a negative relationship in ADHD children. In contrast, increases of serotonin metabolism were negatively related to CB in TS but positively in ADHD patients. In conclusion, when selective information processing abilities reflected by CB start to develop at puberty-onset, there is a relative worsening in ADHD patients. But TS patients show an impairment independent of age. Changes in the balance between dopamine and serotonin systems may contribute to normal and abnormal cognitive development.