Somatosensory information from the rodent whisker pad is relayed via the specific thalamic nucleus to layer IV of the barrel cortex. We imaged this projection in acute mouse brain slices using voltage- and calcium- sensitive dyes (VSDs, CaSDs). VSDs and CaSDs have been shown to detect subthreshold activity and action potentials, respectively. With extracellular stimulation in the thalamus we evoked local signals which propagated to layer IV within about 4 ms. The thalamic signal of the VSD RH1691 was small in amplitude and spread to the cortex without obvious attenuation. The response of the CaSD Fluo-3, in contrast, was large in the thalamus (mean peak amplitude 7% dF/F), but clearly decreased in amplitude in layer IV (17% of thalamic value). To monitor the underlyings of the cortical CaSD response, we blocked excitatory transmission using CNQX and D-AP5. Under these conditions the cortical CaSD response was blocked strongly (mean residual component 8% of control), while the thalamic response was still present (mean 37% of control). These data suggest, that cortical CaSD signals due to thalamic input predominantly represent postsynaptic suprathreshold activity. Incoming action potentials from the thalamus, in contrast, are not detected with CaSDs.