Hello, I'm using the weighted nonlinear regression proposed in nls help. I would like to know which is proper reference to the "Weighted version of Michaelis-Menten model". Which is "The White book" cited in nls help? And, by-the-way, many thanks to Martin Maechler for this function. Best regards, Antonio Olinto Sao Paulo Fisheries Institute Brazil www.pesca.sp.gov.br ------------------------------------------------- WebMail Bignet - O seu provedor do litoral www.bignet.com.br