Secretaría Nacional de Turismo (Palma 468, T494110/441530, Mon-Fri 0800-1300 (phone Mon-Sat 0800-1900 for information).) Good free map (ask for it), information on all parts of Paraguay, but you may need to be persistent. Another map is sold in bookshops. Information on Asunción at http://www.asu-cvb.org.py (Spanish and English). The city of Asunción's municipal web site (http://www.mca.gov.py) also has excellent, up-to-date information in Spanish (eg http://www.mca.gov.py/webtermi.html for bus information).
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