  We have conducted an H-alpha survey of 38 0.77<z<1 galaxies over ~100
arcmin^2 of the Hubble Deep Field North and Flanking Fields, to determine star
formation rates (SFRs), with the near-infrared multi-object spectrograph
CIRPASS on the WHT. This represents the first successful application of this
technique to observing high redshift galaxies. Stacking the spectra in the
rest-frame to infer a total SFR for the field, we find a lower limit
(uncorrected for dust reddening) on the star formation rate density at redshift
z = 1 of 0.04Msol/yr/Mpc^3. This implies rapid evolution in the star formation
rate density from z = 0 to z = 1 which is proportional to (1+z)^{3.1}.
