Intracellular free calcium concentration ([Ca2+](i)) was measured with video imaging in lactotrophs from lactating rats. The median resting [Ca2+](i) was 24 nM (85 cells). The great majority of cells responded to thyrotropin-releasing hormone (TRH) with an increase in [Ca2+](i) (median peak [Ca2+](i) after TRH=298 nM; n=73). In 77\% of these cells this [Ca2+](i) increase was biphasic, with [Ca2+](i) remaining high after the initial peak (median [Ca2+](i) 90 s after TRH application=104 nM; n=56); the second phase depended on calcium influx. Most cells also responded to dopamine (DA), after TRH had been applied. DA reduced or abolished TRH-induced calcium influx and also reduced resting [Ca2+](i) if this was above its initial value. A few lactotrophs responded to TRH only after DA application and withdrawal. We conclude that the population of lactotrophs in lactating rats is heterogeneous, but is not composed of two distinct sub-groups defined by their responsiveness to TRH or DA. Copyright (C) 1996 Elsevier Science Ireland Ltd.