We compute the non-systematic errors of monthly oceanic rainfall over 5¿ latitude by 5¿ longitude boxes derived from data taken by the Special Sensor Microwave Imager (SSM/I) on board the Defense Meteorological Satellite Program (DMSP) satellites and TRMM Microwave Imager (TMI). The mean rain rates between 1998 and 1999 are calculated to be 3.0, 2.85, 2.89 mm day-1 and the non-systematic errors 22.2%, 22.4% and 19.7% for SSM/I onboard the DMSP F13, F14 and TMI, respectively. The non-systematic error for the TMI is smaller than that for one SSM/I at the low rain rates but is comparable at rain rates higher than about 5 mm day-1. The TRMM objective of 1 mm day-1 for non-systematic error is met by TMI for rain rates up to 5--6 mm day-1. Improved space-borne sampling strategy, such as that proposed for the Global Precipitation Mission (GPM), is needed to achieve errors better than 10% at the higher rain rates. ¿ 2001 American Geophysical Union