approximately 3,200 at September 30, 2007. As we continue our sales and marketing efforts, we expect our revenues to increase.
Cost of Revenues
Cost of revenues was $7.6 million for the three months ended September 30, 2007 and $19.1 million for the nine months ended September 30, 2007, compared to $4.4 million and $11.7 million for the same periods in 2006. The increase is due to increased sales volume. Cost of revenues includes adjustment of inventory to lower of cost or market and indirect costs. The per-unit cost to manufacture the OmniPod decreased in the three and nine months ended September 30, 2007, compared to the same period in 2006, resulting in improvements in our gross margin. This decrease is a result of reduced cost of raw materials and increased volumes which improved the absorption of manufacturing overhead costs.
Research and Development
Research and development expense increased $148,000, or 7%, to $2.2 million for the three months ended September 30, 2007 and $1.3 million, or 23%, to $7.2 million for the nine months ended September 30, 2007. For the three months ended September 30, 2007 the increase in expense was primarily attributable to an increase of $275,000 in consulting services partially offset by a reduction in prototype expenses. For the nine months ended September 30, 2007, the increase in expense was primarily attributable to an increase of $700,000 in employee related expenses, $439,000 in consulting services, $137,000 in travel expenses, and $53,000 in tools and other expenses.
General and Administrative
General and administrative expenses increased $1.1 million, or 47%, to $3.4 million for the three months ended September 30, 2007 and increased $3.3 million, or 59%, to $8.8 million for the nine months ended September 30, 2007. For the three months ended September 30, 2007, the increase in expenses was primarily due to an increase of $484,000 in employee compensation and benefit costs associated with the hiring of additional employees, $324,000 in consulting and legal expenses, $158,000 in increased travel expenses, $236,000 in bad debt expense, $92,000 in increased insurance expense, $89,000 in distribution expenses, $53,000 in increased depreciation expense and $49,000 in other expense. For the three months ended September 30, 2006, we incurred an expense of $345,000 related to the disposal of assets, whereas there was no such expense incurred in the same period in 2007. For the nine months ended September 30, 2007, the increase in expense was primarily due to an increase of $1.3 million in employee compensation and benefit costs associated with the hiring of additional employees, $929,000 in consulting and legal expenses, $573,000 in bad debt expense, reflecting increased sales volume, $242,000 in increased travel, $200,000 in increased depreciation expense, $186,000 in increased insurance expense, and $144,000 in other expenses. For the nine months ended September 30, 2006, we incurred an expense of $345,000 for the disposal of assets, whereas there was no such expense incurred in the same period in 2007.
Sales and Marketing
Sales and marketing expenses increased $2.4 million, or 138%, to $4.1 million for the three months ended September 30, 2007, and increased $6.4 million, or 149%, to $10.7 million for the nine months ended September 30, 2007. The increase in expenses for the three months ended September 30, 2007, was primarily due to an increase of $1.3 million in employee compensation and benefit costs resulting from the hiring of additional employees in our sales and marketing department, $510,000 in travel, printing and tradeshow expenses used to support our selling efforts, $338,000 in patient demonstration kits, $187,000 in marketing consultants which include our external trainers and $86,000 in other expenses. For the nine months ended September 30, 2007, the increase in expenses was primarily due to an increase of $2.5 million in employee compensation and benefit costs resulting from the hiring of additional employees in our sales and marketing department, $1.7 million in patient demonstration kits $1.4 million in travel, printing and tradeshow expenses used to support our selling efforts, $604,000 in marketing consultants which include our external trainers and $214,000 in other marketing expenses.