Localized communication networks for office automation, security monitoring, environmental management of buildings, computer communications, and other applications enjoy every increasing demand. This paper considers communication of data and analog message signals over electric power distribution circuits for such applications, using pseudonoise (PN) modulation. Advantages of power distribution circuits include reasonably universal coverage and easy access via a standard wall plug. Disadvantages include limited communication bandwidth, relatively high noise levels, and varying levels of impedance, noise, and attenuation. PN (spread-spectrum) signaling provides immunity to narrow-band signal impairments and casual eavesdropping, multiplexing capability, addressing capability, and easily implemented error rate versus data rate tradeoffs. The primary disadvantage is synchronization delay and hardware cost normally associated with PN code synchronization at the receiver. Our prototype modem avoids these difficulties by using the 60 Hz power line frequency for carrier, code, and data-bit synchronization. Design issues discussed include processing gain optimization, modem impedance specification, carrier frequency selection, and PN code selection. Prototype modem circuitry costing less than $30.00 is described. Measurements on actual power distribution circuits indicate a bit-error probabilityp = 10^{-4}or less at data rateR_{B} = 6kbits/s for single users or atR_{B} = 60bits/s forM \simeq 22simultaneous users. Performance calculations for analog message signals are included.