This work was undertaken to analyse the effects of acute cadmium administration on the pulsatile pattern of prolactin release, in adult male rats. For this purpose, animals were cannulated 40 h before the experiment to allow a continuous blood withdrawal. Two hours after the administration of one dose of cadmium chloride (4.5 mg kg1), the pulsatile pattern of prolactin, during three hours, was studied. The effects of two pulses of thyrotropin-releasing hormone (TRH) (1 microgram per rat), given 60 and 120 min after starting the period of blood sampling, were studied. The mean values of prolactin during the bleeding period and the absolute pulse amplitude were decreased by acute cadmium chloride administration. However, no changes in any other parameters of prolactin pulsatility were observed. TRH administration to control rats increased mean prolactin levels, and absolute and relative pulse amplitudes, but decreased the mean half-life of the hormone. In animals pretreated with cadmium, TRH increased the mean levels of prolactin, and absolute and relative amplitudes of the hormone pulses. No other parameter studied was changed by TRH in cadmium pretreated rats. These data suggest that acute administration of cadmium did not inhibit the pulsatile prolactin release through TRH.