The rate of the electrocyclic ring opening of 7-dehydrocholesterol to previtamin D is investigated by transient absorption spectroscopy with a time resolution better than 300 fs, The dehydrocholesterol, which is a derivative of 1,3-cyclohexadiene, is excited around 267 nm. The primary product, the s-cis,Z,s-cis conformer of the triene previtamin D, appears with a time constant of 5.2 ps. This result is consistent with literature data. We found that it is temperature independent. So there is no activation energy for this electrocyclic ring opening. A barrierless process, on the other hand, is expected to proceed much faster (in about 10(-13) s), unless there is an entropy of activation. The results suggest that the molecule reaches within a few femtoseconds the lowest excited state (2A(1)) of the product, from where it goes on through an entropic bottleneck. The primary product isomerizes thermally to the stable s-cis,Z,s-trans conformer of previtamin D within 125 ps in ethanol at room temperature. This time is much longer than the reported corresponding time in the cyclohexadiene/hexatriene system. We found that it depends on the solvent viscosity and on the temperature. The activation energy was determined to be 15.5 +/- 1.0 kJ/mol.