The effectiveness of a once-weekly i.m. injection of ceruletide (0.8 microgram/kg) in suppressing the symptoms of neuroleptic-induced tardive dyskinesia (TD) was evaluated in a double-blind, placebo-controlled, matched-pairs study. Global evaluation of the severity of TD symptoms over the 8-week study period revealed a significant improvement with ceruletide as compared with placebo. Analysis of the therapeutic response to ceruletide over the course of treatment revealed a slow, but long-lasting improvement of TD symptoms. Side effects, which were mild and transient, consisted mainly of nausea and epigastric discomfort. The incidence of side effects did not differ between the ceruletide- and placebo-treated groups. Ceruletide appears to be a novel and practical treatment that can substantially alleviate the symptoms of dyskinesia.