In an attempt to accelerate the development of formal operations in average young adolescents, up to 30 intervention lessons relating to all formal schemata were given by science teachers, over a period of 2 years, to classes in eight British schools. Boys starting the program aged 12 years-plus showed a pre-test, post-test effect size on Piagetian tests of 0.89 standard deviation compared with control classes; this represents a mean change from the 49th to the 26th percentile in terms of British norms for the development of operational thinking. Middle school students and girls of 12 years-plus did not show greater gains than controls. Gains were shown by girls in one 11 years-plus class and two laboratory classes for the same age group. In one school, gains were maintained for one year after the program ended. There were no effects on tests of science achievement during the intervention. It is concluded that in-service training designed to enable teachers to adapt to the students' increased operational thinking capacity should accompany the interventions. Six tables and four figures are included. (SLD)