At the onset of illumination of chromatophores there was a burst (t1/2 approx. 5 ms) in the rate of the H+-transhydrogenase reaction before establishment of the steady-state rate. The burst was suppressed at high pH with a pK(a) of approx. 8.5. The burst and the steady-state rate were inhibited by either (i) a combination of myxothiazol and carbonylcyanide-p-trifluoromethoxyphenylhydrazone, or (ii) NAD+, or (iii) dicyclohexylcarbodiimide. The results support a model in which substrate binding to H+-transhydrogenase is relatively fast. A subsequent slow step is accelerated by the protonmotive force and a third step, possibly product release, is rate-limiting in steady-state turnover during illumination.