A spectrophotometric model able to compute the integrated spectrum of a galaxy, including the contribution both of the stellar populations and of the ionized interstellar gas of the HII regions powered by young hot stars, has been used to study several spectral features and photometric quantities in order to derive calibrations of the star formation history of late type galaxies. Attention has been paid to analyze the emission of the Balmer lines and the [OII]$��$3727 line to test their attitude at providing estimates of the present star formation rate in galaxies. Other features, like D$_{4000}$ and the equivalent width of the H$_��$ line, influenced by the presence of intermediate age stars, have been considered. Several ways of estimating the star formation rates in normal galaxies are discussed and some considerations concerning the applicability of the models are presented. Criteria have been also studied for ascertaining the presence of a burst, current or ended not long ago. Bursts usually hinder the determination of the past star formation rate.

21 pages, needs A&A l-aa.sty, accepted for publication in Astronomy & Astrophysics