Twelve typical cases of the Rett syndrome and one forme fruste were treated with bromocriptine for six months and then had a washout for two months followed by resumption of the bromocriptine treatment. During the first bromocriptine treatment there were improvements in communication and relaxation in some of the girls: a more regular sleep pattern was observed in 4 and a more varied facial expression in 8, and 4 girls began to utter a few words. The bouts of hyperpnea disappeared in 5 and grinding of the teeth in 3. There was also a reduction in stereotypic hand activities in 5 girls and signs of improved motor abilities in 3. The washout caused a general decrease in the positive effects of the previously administered bromocriptine and resumption of the treatment with this drug led to less marked improvement. Metoclopramide was tested in all the girls before the treatment, and it was noted that, while endorphins were hyporesponsive, prolactin was hyperresponsive. This test was repeated two months after the bromocriptine treatment had been performed and, while beta-lipotropin remained unchanged, beta-endorphin showed increased responsiveness.