To evaluate apomorphine as an indicator of dopaminergic responsiveness, responses to subcutaneous apomorphine, to a single levodopa challenge, and to prolonged levodopa therapy were studied in 100 patients with parkinsonian syndromes (65 with idiopathic Parkinson's disease, 35 with other parkinsonian syndromes). Excellent agreement was found between the initial challenges of apomorphine and levodopa. Apomorphine also accurately predicted the response to long-term levodopa in 90% of the patients. There were 8 false-negative results; 7 were in patients with mild deficits. The apomorphine test is a quick, safe, reliable indicator of dopaminergic responsiveness in parkinsonian syndromes. The test is especially helpful to reassess patients who seem to have become less responsive to levodopa and it also has some predictive value in the assessment of an untreated patient's likely response to dopaminergic therapy.