We conducted a study to estimate the impact that thermal variations and electronic drifts would have on the photometric stability of the multianode microchannel array (MAMA) detectors that are currently flying aboard the space telescope imaging spectrograph (STIS). Included in the present analysis is the expected impact of preferential aging of the microchannel plate due to prolonged concentrations of light in the same location. These data and analyses were used to examine the suitability of specified electronic tolerances to achieve a photometric accuracy of±1&percent;over a period of 1 week, but may be also useful should the spacecraft environment begin to vary. The original parameter tolerances were found to be adequate in most cases, although a few voltages did require somewhat better tolerance specifications. The present analysis indicates the sensitivity of the STIS MAMA detectors are reproducible over long periods of time to an accuracy of 0.62&percent; (0.33&percent; rms and 0.29&percent; systematic uncertainties) at an arbitrary location in the detector format. This result indicates the detectors are capable of obtaining data with a signal-to-noise ratio(S/N)≳200in a STIS2×2pixel optical resolution element, consistent with observed departures found in high-signal laboratory data. Even higher S/N levels have been demonstrated on orbit using techniques that move the spectral image around on the detector. ©1998 American Institute of Physics.